Is it permissible for the accountant to take the 500 dirhams remaining from the sale price of a car, after the dealership has taken its commission and the car owner has received the amount he specified?

If you are the owner of the showroom, then the 500 dirhams are the showroom's commission. If you are an employee there, do not take anything from the car owner except with the permission of your employer, because it falls under the prohibited gifts to employees. If you are neither an owner nor an employee there, then it is permissible for you to take what you agree upon with the car owner in exchange for selling it. However, without an agreement, it is not permissible to take anything, and any surplus amount belongs to the car owner.