First-year math problems: ten people are nine people wrong. Believe it or not, try it!

This is a first-year math problem. It is estimated that it is time to start learning in kindergarten. But never let the problem look simple. It is said that ten people are nine people wrong. Believe it or not, please try it!

Question:
One day, a young man came to Wang's store to buy a gift. The cost of the gift was 18 yuan, and the price was 21 yuan. The young man paid 100 yuan for the gift.
Boss Wang had no change at the time. He used the 100 yuan to change the 100 yuan change to the neighborhood and asked the young man for 79 yuan. However, the neighborhood later found that the 100 yuan was a fake banknote, and Boss Wang had no choice but to pay 100 yuan back to the neighborhood.
 
The question now is: How much did Boss Wang lose in this transaction?