The cashier gave you ten extra dollars. What you do next is worth way more than ten dollars.

Tiny moments with no witnesses are where your standards actually get set, because there is no pressure making the decision for you. Ten dollars is small enough that either choice is easy to justify, which is exactly what makes it informative. You are not deciding about the money, you are deciding what kind of thing you do.

Hand it back quickly, before the internal negotiation begins. Notice that the whole event takes fifteen seconds and stays with you far longer than that. These are the cheapest chances you will ever get to be the person you intend to be.