What is the legal ruling on a friend keeping a sum of money as a "commission" from the price of a car that was sold, even though the sale was accomplished through the effort of the car owner and the friend's role was limited to accompanying and assisting?

"Disputes can only be judged by hearing both parties. However, if this person did not appoint himself as a broker, he is not entitled to a fee unless there is an agreement. The contract of Ju'alah (stipulated reward) is only valid if the reward is known; otherwise, the worker is entitled to a customary reward (Ju'l al-Mithl), as stated by Sheikh al-Islam Ibn Taymiyyah. So, if your friend's work was voluntary, he is not owed anything; otherwise, he is entitled to a customary reward for the work he performed."