Do you take the money back or just grin and bear it? Happened to me and the clerk kinda looked at it longingly and said she was sorry she couldn't help me. I felt bad and let her keep the $20. Should I have just said that I was sorry I couldn't help her, either and taken the bill back? Once before the clerk tossed the bill back at me and asked "What is this?" so the end result was clear, but was conflicted about this last time.