This age old scam was run on my dad’s old business from time to time...they never got me, but from time to time they would nail one of our help....even though we would train some of those kids to watch for this situation.
It’s good to watch out for convenience store clerks themselves, sometimes. Give them a twenty, they give change back for a ten. It happens too often not to be intentional. I’ve gotten so I watch where they put the bill, and tell them “here’s a twenty” beforehand. I’ve had a few that lied and denied it, insisting they got a ten. So, scammers all around.