Do I have to sign a loan modification for my car after it has been paid for if the amount is wrong?

No.

No.

Don't sign. If you sign, you are agreeing with them. Make them PROVE it.

To answer that, I'd need to review all of the loan documents in person.

You do not provide enough information. This modification was it in bankruptcy? You must look at the contract documents.

Can they do something? Yes. People and companies make errors all the time. If you cannot convince them they are wrong, you need to go to court and/or get an attorney to help you resolve the issue. The loan documents probably have an attorney fee provision, which means the prevailing party gets an award of the reasonable attorney's fees incurred. If you show the contract to an attorney, you might get him or her to take the case on contingency " i.e., they don't get paid unless you win or settle". If there is an attorney fee provision and it is clear that you are in the right, you should not have too much of a problem finding an attorney to handle your case on that basis. The thing you need to be concerned about is the tricky language in those loan contracts that may have hidden fees you were not aware of or the deferred amounts bore interest at exorbitant rates, which is why they are claiming the huge amount is still due. In any event, you need someone with more knowledge than you to look it over and give you the advice you need based on the terms of the contract itself.

Call the attorney general consumer division and ask them to investigate.

The ORIGINAL contract should prevail. NO I wouldn't sign. Have an attorney REVIEW the contract and their proposal. If they take your car that could be "larceny by conversion." You could sue them for "treble (triple)" damages. They have to PROVE that there was some error on the original contract. If they can show the error you might have to pay. Absent their PROOF . . . it's their hard luck.

If the amount is wrong then you cannot be forced to sign new documents. However, the loan company may sue for what it claims it is owed and you must defend.

I wouldn't sign it. Leasing and car finance companies are notorious for their tactics. They wouldn't let you modify the payments down, would they?