Ensure payback

I am planning to loan a family member money to rehab a home that he is about to purchase and flip. He has promised to repay the loan plus 5K when he sells the renovated property in 120 days. How best can I legally ensure that he keep his word. I do not want to accept his offer to go on the mortgage with him.... because I would be responsible for the loan should he default. Is there a document that i could get him to sign that would have legal recourse?