Outstanding finance can be a good thing when you come to trade in. First agree a price on what you want to by, then argue the price on the trade in & then hit them there is finance owed & see what they can do about that figure.
Push on every corner, they always have more money to allow than on offer, let them know your there to sign a order that day, only when they are going to let you walk out the door & away you know you have screwed every penny out of him.
Macc