Is the commission he agreed upon with someone to search for a car and sell his friend's car rightfully his, given that the buyer is completely satisfied with the sale and no deception occurred, or should the commission be given willingly?

If a friend asks you to search for a car for him in exchange for a specified commission, and you do so, find the car for him, and he buys it, then this commission becomes obligatory for him. This contract is "brokerage," and it is permissible (shar'i) and must be adhered to. This commission is your exclusive right, and he cannot evade or deny it. The giver's consent has been established by the agreement between you. If he becomes stingy or his soul is not pleased with the money, this is of no value. You can involve some of his acquaintances to collect your right. If he withholds the commission from you, then he is consuming unlawfully (haram), and this sin is not expiated by repentance and seeking forgiveness until he fulfills the right owed. "So, whoever has wronged his brother should seek his forgiveness before his good deeds are taken for his brother, and if he has no good deeds, then some of his brother's evil deeds will be taken and placed upon him."