Denis, you are a stand up guy, clear in your opinion and have met you a few times therefore this is nothing but push back on your opinion and not personal.
The trade value may have dropped but the price of C8s are also dropping exponentially.
I have bought 30+ cars in my lifetime, I don't know the details and don't want to get involved further. From a personal perspective, my trade value would determine my affordability of a vehicle. If the dealer doesn't want to offer the money they have committed to contractually, then it is only fair that the deal be cancelled and monies returned, this is just my opinion. If the value of the vehicle had gone up would the dealership volunteer\offer more money to the customer? Not so sure about that. If the price of the new car has dropped too will the dealership renegotiate the price to the current lower market price? Not so sure of that either. The customer is heart broken as this was a dream purchase for him. I have had terrible run ins with dealerships and it is very daunting and overwhelming to say the least. I hope the customer and dealership get to an equitable conclusion.