They gave you demo models and let you leave the store with them? That seems unusual. Did you buy them, sign a contract, did money change hands?
Here are some issues: They may get demos free but surely they don’t just get as many as they want free. There has to be a limit, like two pairs of a new model, maybe. Maybe he doesn’t think he should have to burn one of his demo models over something that was not his fault. Or maybe he has received all the free ones he can for this model.
OK, so if he thinks you should pay for it, here is another problem: He doesn’t want you to know what he pays for them so he can’t just bill you for his wholesale cost. He bundles services into his price, plus profit, so now if he suddenly says something like, OK, just pay me the $1400 cost then you are going to want to pay that price for the ones you eventually buy. So while it doesn’t sound fair to pay the retail, bundled services price, he has a genuine dilemma here. He might rather lose you as a client than reveal his markup.
Could you find them somewhere else cheaper, maybe over the Internet, that would not have bundled services included in the price, and give that one to him as a replacement? I don’t know if that would work, just a thought.
Unless he can order a free replacement without hurting himself I would say you have a responsibility to replace what you borrowed and lost.
On the other hand, he may pressure you to buy from him with the loss warranty and then “lose” one right off so he replaces it under warranty. First he demands you pay him $3200 for the lost one, then he might come up with the scheme for you to buy two for your son, then pretend to lose one, and he turns it in under warranty. If he does that then you will know what you are dealing with.