that's a thing!! That's a scam people pull. There's a video somewhere on the internet of a man at a convenience store pulling something similar. So the guy buys his stuff and gets his change back, but secretly pockets like half of it and the clerk thinks they messed up and gives him more money. Not the same scam, but similar. When i worked at fast food, people tried all the time to say that I gave incorrect change. Count the drawer, i'm perfect, they leave pissed. XD
My drawer was short ONE time at a job I had and I knew something was up, but it was my very first day and I was training with a manager. Fast forward a month later, the manager was fired for shorting drawers and the safe.