Brokerage is mediation between the seller and the buyer to facilitate a sale. This is permissible in Islamic law so long as the commodity or its like is lawful. This is based on the words of the Prophet , "Muslims must abide by their conditions" [recorded by Ahmed and Abu Dawud].

Imam al-Bukhari reported, "Ibn Sirin, 'Atta`, Ibrahim and al-Hassan did not object to a middleman's fees."

The ruling

Based on this and in reference to the question, brokerage is lawful in Islamic law.

Specifying a commission depends on a mutual agreement between the middleman and his client. There is no prescribed amount of brokerage fees in Islamic law provided it is void of fraud and risk.
Allah Almighty knows best.