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If the both parties knew that the sales price was $28,000 and that the $8,000 was to pay off the loan on the trade in - the paperwork contained a mutual mistake which can be corrected by the seller.
However, if your husband understood that the deal was as the contract states, he can claim that there was no mistake and that the contract is complete.
One issue is that the contract likely states that IF there is a mistake in the paperwork, he'll agree to re-sign the corrected documents. Thus, you need to look at that first.
If he doesn't agree to re-sign, he can expect a fight and will likely be sued. However, there's no guarantee that the dealership will win - however, a judge is not going to like it if the facts show that this was a simple mistake and he's trying to take advantage.