What is the ruling on taking a percentage of the truck rental money in exchange for mediating its rental to a company?

This work is called brokerage, and it is permissible in general, provided that the commission is known. However, if the commission is a portion of the rental amount, it involves uncertainty because it is not known how long the lease contract will continue. Therefore, the commission should be a specified amount or a known fraction of the rental amount for a known period.