You shouldn't take advantage of people for their mistakes.
Mike is on summer vacation and is out and about exploring a new city.
Mike is thirsty and wants to get a bottle of water.
After Mike buys his bottle of water he realizes that cashier gave him too much change back by mistake and returns it.
After Mike buys his bottle of water he realizes that cashier gave him too much change back by mistake but opts to keep it.
The cashier is grateful to Mike for returning the money.
The cashier comes up short at the end of their shift and loses their job.