I don't really understand why you say if you found money on the floor you would hand it in but when a cashier gives you too much change, you keep it. To me it would seem more fair to keep any cash you just find lying on the floor (unless you saw who dropped it) and hand back the extra change to the cashier. Either way, I can't really preach. A while back I used the self service check out at tesco and noticed that in the area under the scanner where the notes are returned to you, there was a fiver sitting in there so I just picked it up with the rest of my change. There was nobody there before me so my guilty conience didn't hang around for long.