Made a stupid mistake. Offered customer a $5 partial refund to accept an earlier edition of a book. He agreed, but I got distracted so failed to take the quantity down to zero on the refund page and ended up refunding him in full -- plus $5!

I followed the prescribed procedure and asked him to contact customer service to have his card charged again. He complied and ended up in a chat with a bot who told him to keep the book and the refund! (He sent me a screen shot of the exchange.)

I did what I was supposed to and the customer was a nice guy. Any recourse?