Our son, E. who is now five, had a twenty dollar bill to spend so we went to the toy store. After looking through all the aisles he finally found something under twenty dollars that he wanted.
To teach him how money disappears from our pockets we always let him hand the money over to the cashier. When the cashier handed E. back his change (3 bills, a quarter, a dime, and a nickel), E. exclaimed, "I got even more money!"
Apparently we skipped a step and need to teach him the values of money.