The seller #1 sells a cap. It costs $10. The buyer wants to buy but he has $25 coin. The seller #1 goes to seller #2 to exchange money. Then he has $10+10+5. The seller #1 gives buyer a cap and the change $15. Later … the seller #2 comes to seller #1 and says that $25 coin was counterfeit and demands to give money back. What’s to be done? The seller #1 gives money back.
Question: how much money did the seller #1 lose?