diff --git a/QRBee.Core/Data/ClientToMerchantResponse.cs b/QRBee.Core/Data/ClientToMerchantResponse.cs
index cb2a8d8..8a41558 100644
--- a/QRBee.Core/Data/ClientToMerchantResponse.cs
+++ b/QRBee.Core/Data/ClientToMerchantResponse.cs
@@ -2,35 +2,15 @@
 {
     public record ClientToMerchantResponse
     {
-        public MerchantToClientRequest MerchantRequest
-        {
-            get;
-            set;
-        }
+        public MerchantToClientRequest MerchantRequest { get; set; }
 
-        public string ClientId
-        {
-            get;
-            set;
-        }
+        public string ClientId { get; set; }
 
-        public DateTime TimeStampUTC
-        {
-            get;
-            set;
-        }
+        public DateTime TimeStampUTC { get; set; }
 
-        public string ClientSignature
-        {
-            get;
-            set;
-        }
+        public string ClientSignature { get; set; }
 
-        public string EncryptedClientCardData
-        {
-            get;
-            set;
-        }
+        public string EncryptedClientCardData { get; set; }
 
         /// <summary>
         /// Convert ClientToMerchantResponse to string to be used as QR Code source (along with client signature)
diff --git a/QRBee.Core/Data/MerchantToClientRequest.cs b/QRBee.Core/Data/MerchantToClientRequest.cs
index 6f843ce..ae8dfaf 100644
--- a/QRBee.Core/Data/MerchantToClientRequest.cs
+++ b/QRBee.Core/Data/MerchantToClientRequest.cs
@@ -4,41 +4,17 @@ namespace QRBee.Core.Data
 {
     public record MerchantToClientRequest
     {
-        public string MerchantId
-        {
-            get;
-            set;
-        }
+        public string MerchantId { get; set; }
 
-        public string MerchantTransactionId
-        {
-            get;
-            set;
-        }
+        public string MerchantTransactionId { get; set; }
 
-        public string Name
-        {
-            get;
-            set;
-        }
+        public string Name { get; set; }
 
-        public decimal Amount
-        {
-            get;
-            set;
-        }
+        public decimal Amount { get; set; }
 
-        public DateTime TimeStampUTC
-        {
-            get;
-            set;
-        }
+        public DateTime TimeStampUTC { get; set; }
 
-        public string MerchantSignature
-        {
-            get;
-            set;
-        }
+        public string MerchantSignature { get; set; }
 
         /// <summary>
         /// Convert MerchantToClientRequest to string to be used as QR Code source (along with merchant signature)
diff --git a/QRBee.Core/Data/PaymentResponse.cs b/QRBee.Core/Data/PaymentResponse.cs
index 0da5777..c41563c 100644
--- a/QRBee.Core/Data/PaymentResponse.cs
+++ b/QRBee.Core/Data/PaymentResponse.cs
@@ -2,23 +2,22 @@
 {
     public record PaymentResponse
     {
+        public string ServerTransactionId { get; set; }
 
-        public string ServerTransactionId
-        {
-            get;
-            set;
-        }
+        public PaymentRequest PaymentRequest { get; set; }
 
-        public PaymentRequest PaymentRequest
-        {
-            get;
-            set;
-        }
+        public DateTime ServerTimeStampUTC { get; set; }
+
+        public bool Success { get; set; }
+
+        public string RejectReason { get; set; }
+
+        public string ServerSignature { get; set; }
 
         /// <summary>
         /// Convert PaymentResponse to string to be encrypted and transmitted back to merchant
         /// </summary>
         /// <returns>Converted string</returns>
-        public string AsString() => $"{ServerTransactionId}|{PaymentRequest.AsString()}";
+        public string AsDataForSignature() => $"{ServerTransactionId}|{PaymentRequest.AsString()}|{ServerTimeStampUTC:O}|{Success}|{RejectReason}";
     }
 }
diff --git a/QRBee.Core/Data/RegistrationRequest.cs b/QRBee.Core/Data/RegistrationRequest.cs
index 3f77c5d..b3a4d05 100644
--- a/QRBee.Core/Data/RegistrationRequest.cs
+++ b/QRBee.Core/Data/RegistrationRequest.cs
@@ -4,35 +4,15 @@ namespace QRBee.Core.Data
 {
     public record RegistrationRequest
     {
-        public string Name
-        {
-            get;
-            set;
-        }
+        public string Name                                   { get; set; }
 
-        public string Email
-        {
-            get;
-            set;
-        }
+        public string Email                                  { get; set; }
 
-        public string DateOfBirth
-        {
-            get;
-            set;
-        }
+        public string DateOfBirth                            { get; set; }
 
-        public ReadableCertificateRequest CertificateRequest
-        {
-            get;
-            set;
-        }
+        public ReadableCertificateRequest CertificateRequest { get; set; }
 
-        public bool RegisterAsMerchant
-        {
-            get;
-            set;
-        }
+        public bool RegisterAsMerchant                       { get; set; }
 
 
     }
diff --git a/QRBee.Core/Data/RegistrationResponse.cs b/QRBee.Core/Data/RegistrationResponse.cs
index 602d479..59d0f49 100644
--- a/QRBee.Core/Data/RegistrationResponse.cs
+++ b/QRBee.Core/Data/RegistrationResponse.cs
@@ -6,23 +6,11 @@ namespace QRBee.Core.Data
 {
     public record RegistrationResponse
     {
-        public string ClientId
-        {
-            get;
-            set;
-        }
+        public string ClientId             { get; set; }
 
-        public string ClientCertificate
-        {
-            get;
-            set;
-        }
+        public string ClientCertificate    { get; set; }
 
-        public string APIServerCertificate
-        {
-            get;
-            set;
-        }
+        public string APIServerCertificate { get; set; }
 
     }
 }
diff --git a/QRBee/QRBee/ViewModels/MerchantPageViewModel.cs b/QRBee/QRBee/ViewModels/MerchantPageViewModel.cs
index 56b7639..d59b077 100644
--- a/QRBee/QRBee/ViewModels/MerchantPageViewModel.cs
+++ b/QRBee/QRBee/ViewModels/MerchantPageViewModel.cs
@@ -50,8 +50,29 @@ namespace QRBee.ViewModels
 
                 var response = await service.PayAsync(paymentRequest);
 
-                //TODO handle response
-                await Application.Current.MainPage.DisplayAlert("Success", "The transaction completed successfully ", "Ok");
+                if (response.Success)
+                {
+                    var check = _securityService.Verify(
+                        Encoding.UTF8.GetBytes(response.AsDataForSignature()),
+                        Convert.FromBase64String(response.ServerSignature),
+                        _securityService.APIServerCertificate
+                        );
+
+                    if (check)
+                    {
+                        await Application.Current.MainPage.DisplayAlert("Success", "The transaction completed successfully ", "Ok");
+                    }
+                    else
+                    {
+                        await Application.Current.MainPage.DisplayAlert("Failure", "Invalid server signature", "Ok");
+                    }
+                    
+                }
+                else
+                {
+                    await Application.Current.MainPage.DisplayAlert("Failure", $"The transaction failed: {response.RejectReason}", "Ok");
+                }
+                
             }
             catch (Exception e)
             {
diff --git a/QRBeeApi/Program.cs b/QRBeeApi/Program.cs
index 8bd8199..61a6167 100644
--- a/QRBeeApi/Program.cs
+++ b/QRBeeApi/Program.cs
@@ -31,6 +31,7 @@ builder.Services
     .AddSingleton<IMongoClient>( cfg => new MongoClient(cfg.GetRequiredService<IOptions<DatabaseSettings>>().Value.ToMongoDbSettings()))
     .AddSingleton<IPrivateKeyHandler, ServerPrivateKeyHandler>()
     .AddSingleton<ISecurityService, SecurityService>()
+    .AddSingleton<IPaymentGateway, PaymentGateway>()
     ;
 
 var app = builder.Build();
diff --git a/QRBeeApi/Services/Database/IStorage.cs b/QRBeeApi/Services/Database/IStorage.cs
index 732932b..cdc4527 100644
--- a/QRBeeApi/Services/Database/IStorage.cs
+++ b/QRBeeApi/Services/Database/IStorage.cs
@@ -38,6 +38,12 @@
         /// <returns>Transaction information</returns>
         Task<TransactionInfo> GetTransactionInfoByTransactionId(string id);
 
+        /// <summary>
+        /// Update transaction after execution
+        /// </summary>
+        /// <param name="info">Transaction to be updated</param>
+        Task UpdateTransaction(TransactionInfo info);
+
         /// <summary>
         /// Inserts CertificateInfo into database
         /// </summary>
diff --git a/QRBeeApi/Services/Database/Storage.cs b/QRBeeApi/Services/Database/Storage.cs
index dbbc0bf..44de1a9 100644
--- a/QRBeeApi/Services/Database/Storage.cs
+++ b/QRBeeApi/Services/Database/Storage.cs
@@ -93,7 +93,7 @@ namespace QRBee.Api.Services.Database
         private async Task<TransactionInfo?> TryGetTransactionInfo(string id)
         {
             var collection = _database.GetCollection<TransactionInfo>("Transactions");
-            using var cursor = await collection.FindAsync($"{{ Id: \"{id}\" }}");
+            using var cursor = await collection.FindAsync($"{{ _id: \"{id}\" }}");
             if (!await cursor.MoveNextAsync())
             {
                 return null;
@@ -108,6 +108,12 @@ namespace QRBee.Api.Services.Database
             return transaction ?? throw new ApplicationException($"Transaction with Id: {id} not found.");
         }
 
+        public async Task UpdateTransaction(TransactionInfo info)
+        {
+            var collection = _database.GetCollection<TransactionInfo>("Transactions");
+            await collection.ReplaceOneAsync($"{{ _id: \"{info.Id}\" }}", info, new ReplaceOptions() { IsUpsert = false });
+        }
+
         public async Task InsertCertificate(CertificateInfo info)
         {
             var collection = _database.GetCollection<CertificateInfo>("Certificates");
diff --git a/QRBeeApi/Services/Database/TransactionInfo.cs b/QRBeeApi/Services/Database/TransactionInfo.cs
index be05600..9339271 100644
--- a/QRBeeApi/Services/Database/TransactionInfo.cs
+++ b/QRBeeApi/Services/Database/TransactionInfo.cs
@@ -41,5 +41,7 @@ namespace QRBee.Api.Services.Database
             Succeeded = 2,
         }
         public TransactionStatus Status { get; set; } = TransactionStatus.Pending;
+
+        public string? RejectReason { get; set; }
     }
 }
diff --git a/QRBeeApi/Services/IPaymentGateway.cs b/QRBeeApi/Services/IPaymentGateway.cs
new file mode 100644
index 0000000..9603997
--- /dev/null
+++ b/QRBeeApi/Services/IPaymentGateway.cs
@@ -0,0 +1,17 @@
+using QRBee.Api.Services.Database;
+using QRBee.Core.Data;
+
+namespace QRBee.Api.Services
+{
+
+    public class GatewayResponse
+    {
+        public bool Success { get; init; }
+        public string? ErrorMessage { get; init; }
+    }
+
+    public interface IPaymentGateway
+    {
+        Task<GatewayResponse> Payment(TransactionInfo info, ClientCardData clientCardData);
+    }
+}
diff --git a/QRBeeApi/Services/PaymentGateway.cs b/QRBeeApi/Services/PaymentGateway.cs
new file mode 100644
index 0000000..0d48c74
--- /dev/null
+++ b/QRBeeApi/Services/PaymentGateway.cs
@@ -0,0 +1,32 @@
+using QRBee.Api.Services.Database;
+using QRBee.Core.Data;
+
+namespace QRBee.Api.Services;
+
+internal class PaymentGateway : IPaymentGateway
+{
+    private readonly ILogger<Storage> _logger;
+
+    public PaymentGateway(ILogger<Storage> logger)
+    {
+        _logger = logger;
+    }
+
+    public Task<GatewayResponse> Payment(TransactionInfo info, ClientCardData clientCardData)
+    {
+        if (info.Request.ClientResponse.MerchantRequest.Amount < 10)
+        {
+            _logger.LogInformation($"Transaction with id: {info.Id} failed");
+            return Task.FromResult(new GatewayResponse
+            {
+                Success = false,
+                ErrorMessage = "Amount is too low"
+            });
+        }
+        _logger.LogInformation($"Transaction with id: {info.Id} succeeded");
+        return Task.FromResult(new GatewayResponse
+        {
+            Success = true
+        });
+    }
+}
\ No newline at end of file
diff --git a/QRBeeApi/Services/QRBeeAPIService.cs b/QRBeeApi/Services/QRBeeAPIService.cs
index 2c0a1bb..01ef060 100644
--- a/QRBeeApi/Services/QRBeeAPIService.cs
+++ b/QRBeeApi/Services/QRBeeAPIService.cs
@@ -1,4 +1,5 @@
 using System.Globalization;
+using System.Runtime.CompilerServices;
 using System.Security.Cryptography;
 using System.Security.Cryptography.X509Certificates;
 using System.Text;
@@ -18,16 +19,18 @@ namespace QRBee.Api.Services
         private readonly IStorage _storage;
         private readonly ISecurityService _securityService;
         private readonly IPrivateKeyHandler _privateKeyHandler;
+        private readonly IPaymentGateway _paymentGateway;
         private static readonly object _lock = new ();
 
         private const int MaxNameLength = 512;
         private const int MaxEmailLength = 512;
 
-        public QRBeeAPIService(IStorage storage, ISecurityService securityService, IPrivateKeyHandler privateKeyHandler)
+        public QRBeeAPIService(IStorage storage, ISecurityService securityService, IPrivateKeyHandler privateKeyHandler, IPaymentGateway paymentGateway)
         {
             _storage = storage;
             _securityService = securityService;
             _privateKeyHandler = privateKeyHandler;
+            _paymentGateway = paymentGateway;
             Init(_privateKeyHandler);
         }
 
@@ -150,13 +153,42 @@ namespace QRBee.Api.Services
             var clientCardData = DecryptClientData(value.ClientResponse.EncryptedClientCardData);
 
             //6. Check client card data for validity
+            await CheckClientCardData(clientCardData);
+
             //7. Register preliminary transaction record with expiry of one minute
-            //8. Send client card data to a payment gateway
-            //9. Record transaction with result
-            //10. Make response for merchant
             var info = Convert(value);
+            info.Status = TransactionInfo.TransactionStatus.Pending;
+
             await _storage.PutTransactionInfo(info);
-            return new PaymentResponse();
+
+            //8. Send client card data to a payment gateway
+            var res = await _paymentGateway.Payment(info, clientCardData);
+
+            //9. Record transaction with result
+            if (res.Success)
+            {
+                info.Status=TransactionInfo.TransactionStatus.Succeeded;
+            }
+            else
+            {
+                info.Status = TransactionInfo.TransactionStatus.Rejected;
+                info.RejectReason = res.ErrorMessage;
+            }
+            await _storage.UpdateTransaction(info);
+
+            //10. Make response for merchant
+            var response = new PaymentResponse
+            {
+                ServerTransactionId = info.TransactionId,
+                PaymentRequest = value,
+                ServerTimeStampUTC = DateTime.UtcNow,
+                Success = res.Success,
+                RejectReason = res.ErrorMessage,
+            };
+
+            var signature = _securityService.Sign(Encoding.UTF8.GetBytes(response.AsDataForSignature()));
+            response.ServerSignature = System.Convert.ToBase64String(signature);
+            return response;
         }
 
         private void ValidateTransaction(PaymentRequest request)
@@ -222,6 +254,31 @@ namespace QRBee.Api.Services
             return ClientCardData.FromString(s);
         }
 
+        private async Task CheckClientCardData(ClientCardData data)
+        {
+            var transactionId = data.TransactionId;
+            var expirationDate = DateTime.Parse(data.ExpirationDateMMYY);
+            var validFrom = DateTime.Parse(data.ValidFrom);
+            var holderName = data.CardHolderName;
+
+            await CheckTransaction(transactionId);
+
+            if (expirationDate <= DateTime.UtcNow)
+            {
+                throw new ApplicationException($"The expiration date: {expirationDate} is wrong");
+            }
+
+            if (validFrom > DateTime.UtcNow)
+            {
+                throw new ApplicationException($"The valid from date: {validFrom} is wrong");
+            }
+
+            if (holderName.Any(char.IsDigit))
+            {
+                throw new ApplicationException($"The card holder name: {holderName} is wrong");
+            }
+        }
+
         private static RSA LoadRsaPublicKey(StringRSAParameters stringParameters)
         {
             var rsaParameters = new RSAParameters