You need to gather all the paperwork that you have regarding this issue and schedule an appointment with an attorney. Mistakes can happen during the excitement of buying or selling a car from either side, but ultimately it boils down to contract law and what the two parties actually agreed to.
I agree with Mr. Downum, this comes down to contract law. I think having a signed copy stating 100,000 miles will help your cause, but this will come down to the paperwork, not oral promises. If they are refusing to pay a claim, you may have real damages, and if that is the case, you need to contact a lawyer, or feel free to contact me so we can look at all the facts and get your situation taken care of.
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