If you are a vendor and someone gives you money for a product, you are not legal bound to give change?

I was told in highschool there was no law about 'giving change back' but I never found info on it. Anyone know? Just curious.


Answers:
If someone presents a vendor with a large bill and the vendor doesn't have change, the customer has the option of either waiving the change or not buying the item. There is no law requiring vendors to have enough cash on hand to make change.
There is no law about giving change back because if you kept the change, it would be stealing, as you never owned their change, they were just allowing you to exchange it for them.