A Quiz

My brother-in-law showed me this quiz. What's your answer? (The English in the original version was pretty bad; I have taken the liberty to correct the grammatical mistakes.)

Richard is selling slippers. His cost is $20 a pair, selling at $30. There was a 20% discount during a certain festive period. One customer came and bought a pair and paid him $50. Richard had no small change so he changed it with his neighbour. The next day, the neighbour came back with the $50 and told him that's counterfeit note. Richard paid the neighbour back $50. How much did Richard actually lose?

For those who love such challenges, I urge you to read a book by Noble laureate Daniel Kahneman: Thinking, Fast and Slow.