A pair of shoe costs $20 and its selling price is $30.

A customer bought a pair of shoe and paid with a $50 note.

Mr Wang didn't have change so he went to his neighbor to break it into small notes.

He returned $20 to the customer.

The next day, Mr Wang's neighbor realized that the $50 note was a fake.

Mr Wang compensated his neighbor $50.

Question:

How much money did Mr Wang lose?