What is the religious ruling on a car, and is it a trust? Is what the friend did permissible, considering the car was registered in his name to facilitate its use, but then he prevented its original owner from accessing it, threatened him, altered it without his knowledge, and then demanded from him the repair costs or that he sell the car to him?

The feigned waiver and registration of properties do not alter their ownership from the original owner. The car still belongs to the questioner, and its presence with his friend is a trust that must be returned upon request, by consensus. Whoever refuses to return a trust is considered an usurper, bears responsibility for its damage, and has no right to demand reimbursement for expenses incurred on it, such as repairs or unnecessary additions made without the owner's permission. As for self-standing additions, he has the right to take them.