The intermediary tricked you into buying a house, saying that it would give you a rebate, but it didn't materialize in the end. You should complain through the consumer association, but you need relevant written evidence. Or for your contract dispute, bring relevant written evidence to court and ask it to honor its promise.
Legal basis: Civil Code of People's Republic of China (PRC).
Article 961 An intermediary contract is a contract in which the intermediary reports to the client the opportunity to conclude a contract or provides media services for concluding a contract, and the client pays the remuneration.
Article 962 The broker shall truthfully report to the client the matters related to the conclusion of the contract.
If the broker intentionally conceals important facts related to the conclusion of the contract or provides false information, which harms the interests of the client, he shall not ask for payment of remuneration and shall be liable for compensation.