All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
This deal has two possible manifestations:
First: The owner asks you to sell the apartment for a defined price, and then you sell it for a higher price. In this case, the price and the extra amount must be given to the owner, because the extra money is a return generated by his property. The broker deserves nothing but a fee. If the seller wants to give him the extra money as a fee, then it is permissible.
Second: The owner asks you to sell the apartment for a definite price and tells you to take the extra money (charged by you). Scholars have differed whether this deal is valid, and thus the agent deserves the extra money, or the deal is not valid and thus the agent deserves nothing but a fee.
Ibn ‘Abbaas and his father, deemed this second type of deal to be valid and thus the agent is entitled to the extra money. None of the Companions opposed Ibn ‘Abbaas all, as indicated by Ibn Hazm, Ibn Qudaamah and other scholars .
This opinion was favored by Imaam Ahmad . It is also preponderant, Allaah willing, based on the aforementioned Fatwa of Ibn ‘Abbaas and his father, the lack of opposition to this from the Companions and because such contracts are basically permissible unless they involve interest or uncertainty.
Other scholars hold this contract to be invalid and thus the agent only deserves the market fee.
Allaah Knows best.