He changed money from the neighborhood and found money again, and then earned 3 yuan by himself. This is as understandable as when the young man paid 100 yuan to find the young man 79 yuan, and the cost was 18 yuan, that is, he earned 3 yuan.

It's just that what happened next confused the problem solver.

After earning 3 yuan, he has to pay 100 yuan to the neighborhood because the paper money is fake, which becomes 3- 100=-97 yuan, that is, the boss loses 97 yuan. So 97 yuan should be paid.