To credit a transaction, you need the transaction ID returned by CCProcessPayment. Pass this (along with other payment info) to the CCRefund function. This is similar to the void process, except it accepts a dollar amount and the credit card number (or the last four digits of the credit card number) used to process the original transaction.
Set Variable [$result; Value: CCRefund( merchantAccount; transactionKey; transactionID; cardNumber; dollarAmount)]
Returns: the transactionID from the payment gateway service if the order is successful, or "ERROR" if there was a problem