Chances are that the dealer's valuation will be considerably lower when the new vehicle arrives. If so, you can simply sell your used vehicle elsewhere and pay cash in lieu of the specified trade allowance. If you are ordering a vehicle that is configured with unusual options that will make it hard to sell, a dealer might reasonably expect a deposit of several hundred dollars. The less you put down, the more leverage you'll have for quick service. Another response if the vehicle you want is not on a dealer's lot is for the dealer to arrange to get what you want from another dealer.
Dealers have ready access to data on the vehicles on other dealers' lots.
Your dealer may simply buy a vehicle from another dealer or may exchange vehicles. The dealer may charge you a little more for a vehicle secured this way than for a vehicle already on the lot. But the charge for this service is usually modest and a dealer exchange often is your best bet. Don't put down a large deposit on the promise of an exchange. The price of the old model may be lower in part because of the price increases that typically occur with each new model year.