I agree. This is so fact specific the contract terms and a chronology need to be evaluated. As a general rule, if a contract is void nothing is owed.
This is not legal advice but only general information. No attorney-client relationship is created without a written and signed retainer. I do not know all the facts of your specific situation, which will affect this general post. You can get more information at my websites: http://kindsvater.com http://internetmarketinglawcenter.com
I'm very sorry to hear about your situation, which sounds very fact specific. As such, I would recommend that you sit down with a local attorney to go over your situation and any paperwork you have. Depending on the contract and applicable law, you may or may not owe the $500. You can use Avvo's Find a Lawyer feature or contact your local bar association for a local referral. I would do this sooner rather than later so you can stop the alleged harassment. Best of luck to you and your husband.
Answer given for general advice and is not a legal opinion, which would require an analysis of the facts and circumstances as well as the applicable law and regulations.
If they rescind the contract I think they can still recover the mileage or for fair use if it is returns. I think it is in the usually in the contract and/or allowed by statute.
This is not a comprehensive answer and it is impossible to provide a meaningful response without a consultation.
The answer to your question may depend on a number of other factors. You might want to talk to a local attorney and have them review the contract and the circumstances surrounding the transaction.