IQ for U

Out of 10 persons, only one manages to get the correct answer.

One day, a man walked into a souvenir shop.
There was this souvenir he was interested to buy.
The price tagged $21, cost price of the souvenir was $18.
The man paid the shop owner with a $100 note.
Not having enough small change, the shop owner went across road to exchange the $100 note with his friend and return $79 to the man.The man left.
It was then discovered that $100 was a faked note.
The shop owner has no choice but to pay his friend with $100.

The Question:
How much the shop owner made loss in this trade?