Is not paying the broker's fee considered an injustice, and what is the evidence for that?

The brokerage contract or mediation falls under the category of contracts that Allah has commanded to be fulfilled, as He says: "O you who have believed, fulfill [all] contracts," and Muslims are bound by their conditions. The broker is entitled to his brokerage fee upon the completion of his work. If a specific fee was not stipulated, then the customary fee is due to him. Whoever refrains from paying his fee is an oppressor and falls under the severe warning mentioned in the Hadith (definition) Qudsi: "There are three whose adversary I shall be on the Day of Resurrection... and a man who hired a laborer and fully took his due from him, but did not give him his wages." As the Prophet (peace be upon him) also said: "Give the hired worker his wages before his sweat dries." Therefore, it is obligatory to promptly pay his fee.