你会怎么算？

友人给了一个IQ题，如下：

一天有個年輕人來到王頭家的店裡買了一件禮物
這件禮物成本是 18元，標價是21元。
結果是這個年輕人掏出100元要買這件禮物。
王頭家當時沒有零錢，用那100元向街坊換了100元的零錢，找給年輕人79元。
但是街坊後來發現那100元是假鈔，王頭家無奈還了街坊100元。
現下問題是︰王頭家在這次交易中到底損失了多少錢 ???

Translation into English:
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?

不小心路过的你请小心的来猜猜吧。
答案不日公布。但友人的答案与我所想的显然不同。