A client refuses to pay the brokerages fees

I am a real estate agent. Recently, one of my clients viewed a floor on my behalf. He came through my advertisement and I arranged his viewing.

After viewing, he took a complete floor directly from management. He denied paying my brokerage fees. I am an agent who makes viewing on the 7th floor in the building and the building security knows very well that no one came before for this floor.

After 4 days, my client closed the deal directly because there is a number mentioned in the management at the building security reception. When I called the management to inform him, he is a key client, one of them replayed: "sorry, we can’t do anything for skipping the agency fees".

I need legal advice on how to take legal action.

Please help!