I don't use cash much anymore myself, but when I did make a purchase with a large bill, I would take note of the last four or five digits of the serial number of the bill that I gave the cashier. That way if there was ever a dispute about which denomination I had given, I could recite the serial number to prove my case. Unfortunately, I have had to use this method a time of two.Sometimes it goes the other way. I was in a convenience store years ago, picking up my drive-home beer (back when one could do that), When the guy in front of me handed the clerk a $10. When he got his change, he told clerk he had given him a $20. Poor old guy went deer in headlights (he had already put the bill in the register).
I told the clerk that the guy had indeed handed him a $10 and he gave correct change.
Scam artist feigned anger, then left.
I got my beer free that day.