Brokerage Discount Not Given By Broker

I had bought a newly launched apartment in Gurgaon. I purchased it through a broker and not directly with the builder. The broker promised to give a brokerage discount of 0.66% of the basic price. I have a letter from the broker on his letterhead. The terms of the letter saying that he will give me a 0.66% brokerage discount once I make 30% payment to the builder as per the payment plan shared by the builder. Now I have made 30% Payment to the builder as per the payment plan in BBA. I reached out to the broker for a brokerage discount of 0.66% and now they are denying the payment to me. What shall I do?