I think we all have come across this scam, I have as a kid while working in my grandfather shop but it is only recently that I discovered that it had a name.
The scammer tries to confuse the cashier by paying for an item with a low price with a large cash bill. Then the scammer gives back the change. He then tries to confuse the cashier with a series of money exchanges. The idea is to get a lot more change than he should get. People like me a little inexperienced kid of seven, then, on the cash register was a perfect target.
Check the video here.
The best thing to do is make sure you simplify the transaction. Tell him you gave me $X. I gave you the item and this amount of change. What helps here is to have cameras pointing on the counter so you both can review the transaction. We sell such cameras.