when i traded in my truck, i owed ~15k on it.
when the salesman asked what i owed, i told him 19k.
he insisted on calling to verify, but i refused until we had a final deal that was in writing.
i kept telling him i needed to get atleast 17k for my truck (to keep the payments where i wanted)
they finally 'caved in' and gave me 16k for my truck.
once they called to verify payoff, they realized that i owed less that i originally said....
they took the extra money that they were giving me for my truck and took it off the price of the new truck.
i just told the guy "oh, maybe i was looking at an old statement". he knew i was lying, but there isnt anything he could do.
it wont always work, but that is how i didnt get screwed when i traded in a vehicle