To perform a refund on Merchant Gateway, follow the steps below:
Log into Gateway and click Transactions in the pane on the left.
Under Transactions, select All from the drop-down menu.
In the list of transactions, click on the Transaction Purchase ID that you'd like to perform a refund on.
The refund button is at the top right, To process the refund, click the Refund button at top right and follow the required steps.