Refunds or credits are performed to reverse a previous sale transaction.  By default, only transactions processed within the Paid system may be refunded.  

Refunds
To refund a transaction, simply enter the Transaction ID and the amount you wish to refund. The amount must be less than or equal to the original amount captured.

What is a Transaction ID?
The transaction ID is the Paid numerical transaction reference ID. This ID can be found within the Transaction Reports section of the merchant control panel.