In reality it is more complicated. Some cashiers take advantage of the situation. They don't register the exchange operations and simply put the difference in the pocket. It is a small fraud that implies big reputational risks to the banks, since a cashier who started with pocketing small amounts of money may later switch to bigger schemes, getting into the clients personal data and accounts. These incidents are also hard to detect, since the bank security service needs to process manually hours of video records from all branches, which will be time consuming and will involve human factor.