Process credits to a credit card
You can void a credit card payment by going to Income -> Invoices and clicking to edit the invoice with the payment you'd like to cancel. In the Payment Detail section, click Void Payment for the payment you'd like to cancel. The system will contact your credit card processor and attempt to cancel the transaction first, if it hasn't posted, and then issue a credit if canceling is not possible.
You can also void credit card payments by going to Income -> Payments -> Void Payments.
If your agreement with your credit card processor allows, you can add direct credit card credits for customers by going to Income -> Payments -> Enter Payments and entering a negative Amount for a CC payment.
You can also issue refunds for customers on existing credit invoices by going to Income -> Payments -> Refund Credit.