if someone hands over $100 for a $25 ticket, they've handed you the $75 change they're due

What?? They handed you 100, not 75. And you're supposed to give out change, not the customer. You can't give back change of 75 when you only have 25 from first customer.

No, they pay you with a 100 dollars bill. And you only have the 25 dollars bill of the previous client.