What is the ruling on the action of the brother who took the car, paid the remainder of its price, then sold it after taking money and a phone from his brother, and did not compensate him for the car or return his money?

What the brother did, selling his brother's car and taking his phone without his permission, is an aggression against the property of others without right. This is impermissible, due to the Almighty's saying: "And do not consume your properties among yourselves through falsehood," and due to the Prophet's (peace be upon him) saying: "Indeed, your blood, your properties, and your honor are sacred." The aggressive brother must return the car and the phone to his brother, and the questioner must repay the debt that his brother paid on his behalf, unless he forgives all or part of it. However, if the brother took possession of the car and the phone with his brother's permission as collateral, then this is permissible and the rulings of collateral apply to it.