If it is an actual mistake, you can hire a lawyer, sue, hide the rig, stomp around all mad and in the end you will lose. By law, mistakes in contracts are always correctable.
Everyone seems to imply that $9000 is no big deal to the dealer. What if the sale price was $10,000 and they made a decimal point error and they contracted it for $1000? That would be a $9000 error, but also a 90% error. What if they forgot to add in a $9000 payoff on a trade? If it is an error in negotiations, fight them. But if they actually made an error, and they can prove it, fighting it is only going to cost you money, time and good will and in the end they will win.