Fixing Seller's house before listing

[Tee Lim]

Hi, I'm trying to convince my neighbor to let me fix her fixer upper house before putting it to the market.

How can I make sure that I get paid once the house is sold from a legal and paper procedure perspective?

Basically this is my proposal:

1. I fix the house and cover all the cost up front (but cost will be split after house is sold)

2. Whatever sold price above current market value will be split 50 50.

Example:

If the house is worth 400k as is today, and actually sold for 550k after fixing and fixing cost 50k, then the extra 100k (after fixing cost) will be split into 2 between the seller and me. So seller make an extra 50k and I make 50k.

I'm not an agent. I'm an amateur investor in LA who have done some flips and do own some investment properties.

Thank you!

Tee