Legal Document for Off-Market Deal

[Garrett Andersson]

I'm currently under contract to buy an off market duplex from a good friend. The house is gutted at the moment and needs $50k in work to get fully rehabbed. I've agreed to purchase the home for "x" price after seller puts $50k into it. For every dollar amount over $50k I'll add it on to purchase price. The goal is I sell my personal house, buy fully rehabbed duplex and house hack it. I'm trying to figure out what legal form or way to put our "handshake" deal into writing. I don't want to put it in the contract because it could waive a red flag with the lender and mess up appraisal/due diligence period. Any ideas or am I overthinking this and it's just a word doc clearly stating everything out and signed by both parties?