Use this call to void a pending charge. The status of the transaction needs to be OPEN, such as transactions that have not settled yet. To refund a transaction that was previously settled, use the Credit method. You will need the Transaction ID of the charge to be voided.

{
  "Transaction_VoidResult": {
    "ErrCode": 0,
    "ErrMsg": "",
    "FunctionOk": true,
    "RespMsg": "Successful Transaction Void TxID : 53 [097706]",
    "TxApproved": true,
    "TxID": 53
  }
}

private void RESTTransactionVoid(string SessKey, int TxID)
        {
            /// Session Key gathered using the Authenticate Command.
            SessKey = "70826FC89809414BB4303231333441303331343335";
            
            // transaction you need to VOID
            TxID = 1;


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

            // create Request 
            WebRequest request = WebRequest.Create("https://easypay5.com/APIcardProcREST/v1.0.0/CardSale/Void");
            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_VoidResult.FunctionOk;
            int ErrCode = (int)Result.Transaction_VoidResult.ErrCode;
            string ErrMsg = (string)Result.Transaction_VoidResult.ErrMsg;
            string RespMsg = (string)Result.Transaction_VoidResult.RespMsg;
            bool TxApproved = (bool)Result.Transaction_VoidResult.TxApproved;
            int respTxID = (int)Result.Transaction_VoidResult.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;
            }
           
            // The return Authorization may not be approved  , check the Response message for reason.
            if (!TxApproved)
            {
                MessageBox.Show(RespMsg.ToUpper() );
                /// important to insert your Logging function here 
                return;
            }
           
            //Successful Void 
            MessageBox.Show(RespMsg );
            /// important to insert your Logging function here 
            return;
        }