Question

Suppose you give a customer change back for a $20, but she had given you a $50. she is a regular customer and will be returning to the store later in the day.

Answer 1

You should tell your manager because the manager probably knows the person if the person visits the store all the time. Then when the person comes back, you can give her back the money, that is, the manager can do it, and your manager can say that you found it and that you're a good and an honest worker.