In the name of Allaah the Most Merciful the Ever Merciful.
As-Salaamu ‘Alaykum,
I own a real estate office. The owner of a house came to me in search of someone to buy it. He literally said to me, ''Bring me 150,000 and then look after your own benefit.''
After a while, another broker and I found someone who would buy the house for 180,000. I told the owner that the other broker wanted the difference of the price which is 30,000, and he agreed. After receiving the whole sum, he did not fulfill his promise. Is this lawful or unlawful?

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 ‘alayhi wa sallam, is His slave and Messenger.

What you and the other broker did is called brokerage, which means searching for a buyer for a certain commodity, acting as an intermediary between the seller and the buyer and facilitating the transaction. Brokerage is lawful according to the predominant opinion of scholars.

Ibn ‘Abbaas said, “It is permissible to say, 'Sell this garment, and any amount that exceeds such and such a price is yours.’” [Al-Bukhari as a Mu‘allaq Hadeeth; other scholars narrated it as a Mawsool Hadeeth]

Consequently, if you agreed with the owner of the house to sell it at a given price and that you are entitled to the difference in price, and you and the other broker promoted the house until you found someone who bought it for the price that you mentioned, then you and the other broker are entitled to the difference in price. The owner is not permitted to breach his promise and waste your effort. Allaah The Almighty Says (what means): {And fulfill the covenant of Allaah when you have taken it.} [Quran 16:34]

The Prophet, sallallaahu ‘alayhi wa sallam, said: “The characteristics of the hypocrite are three: When he speaks, he tells lies; when he gives a promise, he breaches it; and when he is entrusted with something, he betrays the trust.” [Al-Bukhari and Muslim]

Allaah Knows best.