Is it permissible for the guardian of his intellectually incapacitated brother's money, who is authorized to sell cars on installments, to sell a car to himself on installments, knowing that he has legal guardianship over him?

It is permissible for a guardian to sell his minor brother's car to himself on an installment basis, provided that this does not lead to a squandering of the minor's money, and that he acts in a way that preserves and secures the minor's rights, such as seeking the highest price, having witnesses to the transaction, and documenting the sale with official papers. The Hanafis permitted this if it benefits the minor, just as Ibn Taymiyyah permitted a guardian to purchase from an orphan's property if he offered more than its auction price.