I happened to be at a medical store just two days ago when the girl at the cash counter took the money I handed over to her and returned the same amount of the bill instead of the change.
Obviously, there was some confusion at the counter, so instead of thirty plus, I received seventy plus in change. I was in a real hurry and left immediately but on realizing that an error had occurred, went back and returned the money.
A few years ago too, I had done a similar thing though I had to go a little far to return the extra dress (not money) that I had received at a textile shop the previous day.
If the employees lose money or things to customers this way, the money is generally deducted from their salary, so I would never think of keeping it once I realize that I have received more than what I had to get.