Okay first.
You say they wrote him a receipt but did he take the pieces?
If not then you can nullify the deal and give him his money back. The receipt is meaningless as it is not a contract, just a receipt. He can scream all he wants but again it is meaningless. ie... if a store advertises a product for $xx.xx but it is really worth $xxxx.xx they do not have to sell it to you for $xx.xx.
I was in upper management of a national retail chain for 25 years and watched many a customer get pissed off when they wanted us to feel obligated to give them a misprint or something off a sale rack that wasn't supposed to be there.
Second
Verbal agreements are binding in many cases, especially if there is some relationship between the individuals. Go to court!
Third
Call the FBI as they have a fraud division to help individuals recover monies or properties from just such a deal. If you get scammed on e-bay or such they help you. You may have to go to your local police first for some reason that escapes me before the will FBI get involved in some cases.
Fourth
Go to court and get the judge to force the buyer to ante up the rest or give it all back (in good condition) and you refund his money.
This will be the hardest to accomplish as only about 7% of judgements are ever collected on even after the judge rules in your favor.
Mikey