Answer: It is either another 5 dollar bill or 5 ones. It can be both.
Step-by-step explanation: Cause if he has 20 dollar bill and he gives him a 10 dollar bill and a five dollar bill he would have 15 dollars, so the cashier could give him a five dollar bill or five ones. But if you make it realistic, is someone going to take their time and give you 5 one dollar bills, or just one 5 dollar bill. You pick.
Hope this helps.