Seems to me there's a lot of "Do the honorable thing" bite the bullet..stuff going on here. Just because OP accepted a deal and now don't want to find it's changed sure don't make him a bad guy.
My take is that the dealer contacted the bank,got the wrong figure or wrote something down wrong, the bank or the dealer screwed the pooch on this one..
If the OP don't want to bite this bullet he can say ok..everybody back to square one deals off... bet a bunch the dealer will decide to let the deal stand.
OP Gets the old unit back, the dealer gets his back, and the bank gets their money.
Nobody's being dishonerable by doing this. a deal is a deal. I don't get the payoff figures from my bank, the dealer does... OP dealt in good faith, somebody made a 1500 dollar OHS**T !!...
Me thinks the bank and the dealer have a business agreement, the bank holds the dealers paper.
I'd call a halt and start over....with another dealer and maybe bank too....