Is a commission due to the e-commerce website if a car was exchanged for another, along with an additional payment, and the other car was sold later?

If the selling price of the car is known and the website stipulates a percentage of it, then there is no harm, and you must pay the percentage. However, if the price is not specified and they stipulate a percentage of any price, then that is an invalid contract according to the majority of scholars, because the fee must be known. Some scholars, however, deem it valid. According to the majority's view, the customary fee must be paid.