Ones in a while

At a grocery store I watched the customer ahead of me at the checkout stand. She divided her groceries into two transactions.  After she paid cash for the first transaction, she shuffled the change with the other bills in her hand.  Attempting a con, she told the cashier that she was owed $8, but only received three ones.  While the cashier rang the second half of her groceries, he thought about the change carefully and said, “Oh, I do remember giving you $8 . . . but I also remember giving someone $4.”