Daylight Robbers

I paid a $7.75 sandwich with a $20 bill and 75 cents. They gave me back 3 dollars. Mid-meal I realize that and go back for my missing $10, they say they will call me when they close their business day and count their registers. They never called and the next day manager wasn't available.
I consider that their lack of showing good faith by returning my call makes them guilty of stealing $10 from a customer.