How Much Money Did The Shoe Store Lose?

This was a small group activity that was rather interesting...very few groups came to the correct solution.

A man went into a shoe shop to buy a twelve dollar pair of shoes. He handed the clerk a twenty dollar bill. It was early in the day, and the clerk did not have any one dollar bills. He took the twenty dollar bill and went to the restaurant next door, where he exchanged it for twenty one dollar bills. He then gave the customer his change. Later that morning, the owner of the restaurant came to the clerk and said, “This is a counterfeit twenty dollar bill.” The clerk apologized profusely, took back the fake bill, and gave the restaurant owner two good ten dollar bills.

Not counting the cost of shoes, how much money did the shoe shop lose?