The teller counts of three hundred dollars - or 3 thousand- big difference- The man knew it was an error - He should have told the teller then and there- Cut it left, right, spin it top to bottom or bottom to top- It is still theft. The money was not his - he knew who the rightful owner was. He stole it.
So if someone gives you money but didn't mean to, it's theft?
What if a restaurant forgets to charge you for an item on your meal - what do you do? That would be theft, according to you, because the restaurant gave you something.
He didn't steal the money. The teller gave it to him by mistake. He didn't actively take the money, he accepted what was given to him.
Try as hard as you like, you'd never make a theft charge stand up in court.