Not a done deal, I don't think anyone's said that. but it can be an agreement to perform further acts.
I don't think there's any doubt that this seller agreed to provide scans to him and agreed to the buyers price and that, if the back cover scan was acceptable, that the deal was done.
Those extra acts he agreed to perform were an agreement unto themselves.
Buyers buying a house can place contingencies on a contract that allows them to walk away clean...that's still a binding contract.
Here it's not so complicated. He agreed to do something and failed to do so. Cut and dry.
Personally I still come back to the fact the buyer really has no skin in the game here. In saying that I don't think it was "right" for the seller not to provide the scans they said they would provide and it seems like the answer is they just took more money.