Is my brother's sale of the car considered Islamically valid, and is he obligated to compensate the buyer after the buyer discovered that the car's roof had been replaced, knowing that my brother was unaware of this and sold it due to his need for money?

If a person buys a car and discovers a defect in it that diminishes its value, he has the option to either return it and receive the full price, or keep it and take compensation for the defect, known as "al-arsh" (indemnity). This option is established whether the seller knew about the defect or not. Al-arsh is the difference between the value of the car when sound and its value when defective, and it is calculated based on the original price at the time of the contract. Your brother may demand al-arsh for the defect from the first seller. Shaykh al-Islam Ibn Taymiyyah was of the opinion that the buyer has no option to take al-arsh except with the seller's consent; rather, he must either return the item or keep it without compensation, unless the seller was fraudulent.