These are all fine answers. But the problem is, the invoice was $10. They paid $12 via ach on that invoice. When I put the payment towards that particular invoice, they have a -$2.00 balance. If I create a $2.00 credit memo for them, they then have a -$4.00 overall balance, $2.00 from the overpayment on the invoice and $2.00 from the credit memo. How to give them a credit memo but not double the credit on their overall balance???