Presented a deal not sure best way to protect myself

[Devon Remington]

I have a friend of mine that ive known for a few years and weve talked about doing deals together but we never made it happen. hes currently in the process of building a house in the greater salt lake area that he has under contract for 750K and current market values of the house once its done put it around 950k - 1.1m because of the rise in interest rates his estimated payment has increased almost $1000+ per month and thats more than he wanted, so his plan is to get the house immediately put it back up for sale and collect his roughly 200k+ in equity and buy something with a lower payment.

Now, where i come into play is he is wanting some money to help with down payment and potential holding costs. hes wanting 50k and he'll pay me 18% ROI his intentions is not to have the money for more than 3-6 months so I would make roughly 10K for letting him use the money for 3-6 months. does this sound like a reasonable deal? and how would i structure it (contract or the like) to make sure were both clear on amount being borrowed repayment plan, and penalties if he needs to hold the money for longer? etc. any insight appreciated TIA.