Awesome story.
I hate people like that.
Just last night I rang up some guy while I was on the phone with another store. The total was 13 something or other and he gave me 14 dollars. Bastard comes back 15 minutes later telling me he gave me a 20 and I owe him money back. I guess he thought I was distracted and didn't remember what he gave me so he'd get some free money.
I kinda thought that maybe he was honestly mistaken and didn't remember, but when I give you change back in coins when it should be in bills, wouldn't you notice right then?