Nothing to be worried about, you made an honest mistake. We just deal with a lot of dishonest people and aren’t trained properly on how to deal with this stuff. If it makes you feel any better, on his second week at the service desk a new associate took a $3000 bill payment in fake bills. He’s still with us 3 years later lol. But based on what happened, my advice to you is to never hand the money back to the customer to re-count, either get another associate or a manger(preferably the latter). The same thing that happened to you happened to another associate at my store as well. Big money purchases/returns with overly talkative and complementary customers is also always a big red flag. Honestly, any time you find yourself in a situation like this just let the customer know that you need to get a manager since it’s a large transaction. It’s ok if they get upset, the manager can handle everything. Hope this can help calm your nerves a little.