Seller and cap

The seller sells a cap which costs $10. The buyer comes up, tries the cap on and agrees to buy it, but he has only $25. The seller sends the messenger boy to the neighbour to get $25 exchanged. The boy comes back and gives $10 + $10 + $5. The seller gives the buyer the cap $15 change. Right after that comes the neighbour and says that the bill was fake, and demands to give his money back. The deceived seller can do nothing but return money to his neighbour.

So how much money did he lose?