Builder looking for investment partner. How can I lock in a 'finder's fee'?

[Albert Arguelles]

Hello everyone! Happy Monday! Got a call this weekend from an old friend that his boss, a custom home builder, is looking to develop land and sell lots. He needs someone to invest $330k to match his investment. I have two other big players in mind for him. My question is, how do I assure a finder's fee? What document should I use? And what would be a fair fee for bringing him a potential investor? Thank you so much in advance! Albert Argüelles