Scheme

I used to open the YWCA gym at 6:30 every morning. It sucked but the extra money was good and I could get a jump on the news of the day.
 
A guy came in asking for a $10 bill in exchange for two $5 bills. I thought it was weird and that he might be trying to rob me. It wasn't my money in the register and it was barely 7 am so whatever.
 
I gave him a ten and as I was putting the fives in the register he did something weird with his hands and slapped one on the counter. 
 
"You gave me a one," he said.
 
I thought it was weird but didn't care because it wasn't my money. I took the one and gave him (another) $10.
 
He gave me $11 in total and I gave him $20. I checked the register and we were down $9. It seemed like a strange way to make $9, but he was getting the same amount of money in 60 seconds that I made in 60 minutes.
 
I wrote an e-mail to my boss and went about my business.