You can record a credit card payment from a customer on a posted invoice in Sales/Receive Credit Card Payments. To apply the discount, first post a Credit Memo in Sales/Invoices by following these instructions.
1. Open Sales/Invoices.
2. Tab to Trans. type Credit Memo.
3. Enter customer Serial number or name.
4. Apply to the original invoice number.
5. Tab to Invoice Details section and enter Discount in the Description or Item ID if configured. Enter Discount amount which should be converted to a negative amount and distribute to the Discount GL account in the GL distributions tab.
6. Leave the Payment tendered section blank and post.