Wizardry
Some people are really talented when it comes to scamming others. They probably spend hours practicing their skill because how else could they pull off stunts like this? The OP worked at a small convenience store that also processed money orders. A guy walked in for a money order worth $1,000.
When the OP counted it, he realized he was $20 short and pointed it out to the guy. He took the bills back, flipped through them, and added the $20. The OP absentmindedly forgot to double-check and only realized when his bosses told him the next day that they were short by $500. Wait, what? Apparently, the scammer had pulled some sleight of hand wizardry and duped the store of $500.