finders fee / referral fee legality?

[Robert Griffin]

Lets say that I am a real estate salesperson in the state of California. I am putting out the word that if someone brings me a client and it goes into escrow, then the person who came to me with the client gets a negotiated fixed rate (example: 500 bucks). Is that legal? Would i have to have something in writing and signed? Full disclosure? Greatly appreciate some replies, thanks.