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I am sorry to hear about this situation. On this website, I do not always get to give good news, and this is one of these times. I understand that hearing things less than optimal is not easy, and I empathize. Thanks in advance for not "shooting the messenger."
Unless your brother has mental difficulties, then this would be a binding contract. Just because your brother is handicapped does not mean that he gets special consideration to get out of a contract. And just because he is illiterate also does not mean the same thing. In short, if he voluntarily entered into this contract and whatever they discussed verbally is reflected in the contract (there may be an argument to make if the dealer changed the documentation when promising a lower price but actually signing him up for a higher one - let me know if you think this happened), then he is bound by the deal.
The fact that he should not have qualified does not mean the contract is void, either. A lender has the right to sell a vehicle to someone with low credit.
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