Use this call to process a refund to a settled charge. You will need the Transaction ID and the amount to be refunded.

{
  "Transaction_ApplyCreditResult": {
    "ErrCode": 0,
    "ErrMsg": "",
    "FunctionOk": true,
    "RespMsg": "Successful Credit Pending Transaction ID : 000057",
    "TxApproved": true,
    "TxID": 57
  }
}

private void RESTTransactionCredit(string SessKey, int TxID, decimal CreditAmount)
        {
            /// Session Key gathered using the Authenticate Command.
            SessKey = "70826FC89809414BB4303231333441303331343335";

            // transaction you need to refund
            TxID = 1;


            string json = JsonConvert.SerializeObject(new { TxID = TxID, CreditAmount = CreditAmount });
            byte[] data = Encoding.UTF8.GetBytes(json);

            // create Request 
            WebRequest request = WebRequest.Create("https://easypay5.com/APIcardProcREST/v1.0.0/CardSale/ApplyCredit");
            request.Method = "POST";
            request.ContentType = "application/json";
            request.ContentLength = data.Length;

            // use your session key obtained when you authenticated 
            request.Headers.Add("SessKey: " + SessKey);

            string Myheaders = request.Headers.ToString();
            string responseContent = null;

            // Using the Try block will catch communication errors 
            try
            {
                using (Stream stream = request.GetRequestStream())
                {
                    stream.Write(data, 0, data.Length);
                }
                using (WebResponse response = request.GetResponse())
                {
                    using (Stream stream = response.GetResponseStream())
                    {
                        using (StreamReader sr99 = new StreamReader(stream))
                        {
                            responseContent = sr99.ReadToEnd();
                        }
                    }
                }
            }
            catch (Exception ee)
            {
                MessageBox.Show("Problem communicating with EasyPay Service:" + ee.Message);
                /// important to insert your Logging function here 
                return;
            }

            // Check for null Response as this would be a critical communication error as well
            if (responseContent == null)
            {
                MessageBox.Show("Critical Error!");
                /// important to insert your Logging function here 
                return;
            }
            /// develop Response Object 
            var Result = JsonConvert.DeserializeObject<dynamic>(responseContent);
            bool FunctionOk = (bool)Result.Transaction_ApplyCreditResult.FunctionOk;
            int ErrCode = (int)Result.Transaction_ApplyCreditResult.ErrCode;
            string ErrMsg = (string)Result.Transaction_ApplyCreditResult.ErrMsg;
            string RespMsg = (string)Result.Transaction_ApplyCreditResult.RespMsg;
            bool TxApproved = (bool)Result.Transaction_ApplyCreditResult.TxApproved;
            int respTxID = (int)Result.Transaction_ApplyCreditResult.TxID;
         
            //Check for unexpected Errors on cloud servers. If errors found Stop Processing and check ErrorCodes
            if (!FunctionOk)
            {
                MessageBox.Show(RespMsg + " " + ErrCode + " " + ErrMsg);
                /// important to insert your Logging function here 
                return;
            }
            //Refund not approved. Pleasse check the response message.
            if (!TxApproved)
            {
                MessageBox.Show(RespMsg);
                /// important to insert your Logging function here 
                return;
            }
            //Successful Refund shows the TransactionID
            MessageBox.Show(RespMsg + ": TXID : " + respTxID);
            /// important to insert your Logging function here 
            return;
        }