Your only legal hope here is to asset that there was an oral contract for the sale of the vehicle and that your husband met all the terms of the contract and that now his aunt and uncle are in breach of the contract for failure to deliver the title to the vehicle.
However, unfortunately since the contract is entirely oral then the only proof of the contract will be tesimony from someone about the conversations with the deceased. There are evidence rules that prevent anyone from entering into court conversations with the deceased. This is going to prevent you from proving that the contract existed in court. Because you cannot prove the contract then you have no way to hold the aunt and uncle to any terms of the contract, including arguing the disputed term of the $2,500 down payment.
I would have to say that it sounds like your best bet may be to try and negotiate with them something you can live with, pay them and move on with this. Since this was not put in writing there is really no legal ground to challenge their assertions. In fact, they could deny a contract completely and you would be without the entire car.
I cannot provide you with legal advise. I have provided you with information about the law related to your question. My answer, and any information that you find online, should not take the place of having a consultation with a lawyer in your area to advise you regarding your specific issues.
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