How can I ensure getting a referral fee for finding a tenant buyer?

[Jen Anderson]

I had recently met somebody who said they were looking for a 4 or 5 bedroom house on a contract for deed in Midtown area MNPLS and that they had 10k to put down. If I do find an investor who has a property available in that area-and it could legitimately be a deal, how can I make sure I get a referral fee from them (investor) other than just trust and a handshake and building a relationship? i.e. what kind of contract?

P.s. Aside from me finding out how much he can afford monthly, is there anything else I should do, or find out?