A teddy bear has a cost price of $18, and a selling price of $21. One day, a young man decided to buy that bear and gave the shopkeeper a $100 dollar bill.

Unfortunately, the shopkeeper do not have $100 change. Hence he went to change the $100 bill into smaller change from a passer-by and gave the change to the young man.

After some time, the passer-by realise the $100 is a fake and demanded his money back from the shopkeeper! The shopkeeper has no choice but to return $100 to the passer-by.

Question: how much exactly did the shopkeeper lost?