I would be more concerned that the victim will discover the discrepancy and report it to the card issuer as a fraudulent charge. Since it was on a credit card, which you appear to have access to in some sort of a "merchant" role (perhaps you're a server, a salesperson, etc), I think if I were in your position, I would contact the victim and say "I apologize, I made a mistake on your invoice, and I am putting through a $50 credit on your card". Since you are truly remorseful, I certainly would NOT say it was intentional, just that it was a mistake - believe me, having worked in retail for over 20 years in a service and goods industry, honest mistakes do happen with charge card orders, for reasons as simple as someone misread another person's handwriting and punched in a 3 as an 8, to weird glitches in the card reader that make it come back with a wrong total upon authorization.