Just contract negotiations. But of course it can be that if you don’t agree, they won’t sell. Thus it forces the perspective buyer to not be able to buy unless they agree.
For an unpopular model with lots of stock, these things may not come up. For a popular model, take it or leave it. That’s the problem. What’s the recorse? What’s the plan b? Good luck finding a dealer somewhat close willing to buy it.
Of course most all of this is moot if someone is a serious buyer that wants to keep the vehicle.