Is it permissible or forbidden for the homeowner to break his promise to pay the difference (30,000) after he had requested a specific price (150,000) and the house was sold for a higher price (180,000)?

The acts of brokerage that you and the intermediary performed, which involve finding a buyer for a seller or a seller for a buyer and mediating between them, are permissible according to the most preponderant opinion of scholars. If you agreed with the seller that you would sell the house for a specific price and that any excess over this price would be yours, then you sold the house for the mentioned price, the excess belongs to both of you. It is not permissible for the homeowner to break the agreement or renege on his initial word, based on the Almighty's saying: "And fulfill the covenant; indeed, the covenant will be inquired about," and the Prophet's (peace be upon him) saying: "The signs of a hypocrite are three: when he speaks, he lies; when he promises, he breaks his promise; and when he is entrusted, he betrays."