Change Is Possible

John asked the store clerk for change for a $10 bill. "I am not able to open the cash drawer unless you actually make the purchase," responded the clerk.
"That's not true," replied John.
"It is true," maintained the clerk.
Outside of pointing out the obvious, that the cash drawer has to be opened for such purposes as filling it at the beginning and emptying it out at the end of the shift, what can John actually do to prove to the clerk that he can open the drawer if he chooses?