A seller sells a hat for $10. A buyer buys this hat and gives $20 banknote to a seller. The seller does not have the change, he sends his son to change the $20 banknote at a neighbor's store. The boy brings back 2 banknotes of $10 and the seller gives to a buyer $10 of change. Later this day the neighbor comes and shows that the $20 banknote is fake, and requests his money back. A seller gives $20 back to the neighbor from his pocket. Calculate what is the total loss of a seller, considering the value of the hat as $10. Also explain how you calculated.