If you are voiding a transaction or issuing a refund by using the Quick Credit option in your payment gateway, you are sending data back to the card association(s) for a new transaction. Therefore your cost is a transaction fee.
**Note: You are not refunded for the discount rate paid on the original sale. It is popular belief the discount rate is the card association’s commission, when in actuality the fee is for using the network and transferring funds. The action of accessing the network does not change just because the cardholder is getting their money back.