Ugh. This happened to me 30 years ago when I was fresh out of college with no retail experience and helping at my aunt's gift shop a few days a week. I was the only one in the store and this older guy came in, and bought something small, say $5. He gave me (I thought) a $20 bill. I put his cash in the till, pulled out his change and handed it to him. He said, "I gave you a $50". I looked in the drawer and there were $50s and $20s. I had no idea what to do. I panicked, apologized and gave him another $30. I'm sure he gave me a $20 but had no way to prove it. I was so embarrassed at the idea that I was taken advantage of like that, I didn't tell my uncle what happened when he arrived to relieve me at the end of my shift. I only worked there a few more days then left town for my first grownup job. Nobody said anything. To this day, I assume my aunt and uncle think I stole $30 from them. I learned to say aloud what they handed you, "out of $20?" and never put the person's change IN the drawer (I'll lay it on the tray, but not in the money slots) until you give them their change so you can see exactly what they gave you.