Hello I have a question and it is about an ATM in pseint

I have to make a program that allows me to make a charge to an ATM with $ 50, $ 100, $ 200 and $ 500 bills. The doubt is that when a user realizes an extraction that is multiplo of 50, how do I discount the tickets that he extracted? And how do I make an error if I put another multiplo that is not 50. Greetings and thanks