The pharmacist's revenge

Bush's Pharmacy was across the street from the home where Pops grew up. It was where his family bought all their medicinal wares in the Forties and Fifties, a typical neighborhood drug store. As such, it sold products other than prescription drugs and over the counter remedies for your ills. Among those products were cigarettes. There of course was not the public disdain for smoking as there is now, so it meant nothing that your pharmacist sold smokes.
Late one Sunday a customer walked into the store and asked for a pack of cigarettes. Mr. Bush reached into the display and handed the man a pack, which happened to cost twenty five cents. The guy opened it, took a cigarette out, and lit it; again, it was no faux pas to smoke in a store back then. Next he drew his wallet out of his pocket and offered Mr. Bush a twenty. A Canadian twenty. "It's all I've got," he explained smugly. What he was trying to do was get American money in place of the Canadian without the trouble of going to a bank or currency exchange.
Mr. Bush didn't say anything. He merely reached under the counter, took out an old cigar box, and gave the man $19.75.
Canadian.
It seemed Mr. Bush had accumulated a bit of Canadian cash over the years and had kept it for no particular reason.
But things happen for a reason. In this case, to teach a smart aleck a lesson.