Merchandise store joke

A counterfeiter decided that the easiest way to pass off his phony $18 bills would be to unload them in some small rural town, so he drove until he found a tiny town with a single general merchandise store.

He entered the store, went up to the counter, and handed one of the bogus bills to the man behind the counter.
"Could you change this for me, please?"

The store clerk looked at the bill for a few seconds then smiled at the man.
"Of course I can. Would you prefer two $9 bills or three $6 bills?"