My dad does that a lot with the $2 bills. It’s lots of fun watching their poor little brains short-circuit when spending them.
If your dad wants to have even more fun with $2 bills, have him get a bunch of brand-new bills with consecutive serial numbers from the bank and make a money pad with them. All he needs to do is to put the bills on a piece of thin cardboard the same size, then apply rubber cement and surgical gauze to one end of the bills, folding it under the base for about a quarter of an inch. Then when he buys something, he can whip out the money pad and peel off the bills.