Is the money remaining from the sale of the car lawful if the broker took 10% of the sale value?

It is permissible to sell your car to your friend for a percentage as commission, even if the commission is unknown. This falls under the category of "Ja'l" (reward/stipend), where the unknown nature does not cause harm according to some jurists, as long as it does not prevent delivery. You are permitted to benefit from the remaining amount, as it is the price of your car. However, for those who do not permit unknown "Ja'l," you must give your friend the customary wage (ujrat al-mithl), and the rest of the price will be yours.