Post-close partnership with seller to split flip profit.

[Joshua Johnson]

I'm looking at purchasing a property that is an area with low volume, and is very tough to comp to homes that have sold nearby. It's a property that with the most direct comps that are available, is probably an ARV of $250k, however, most of those comps aren't fully renovated, and are more plain/boring homes, while this home is a 100+ year old, historic Victorian that I believe can sell for quite a bit higher potentially, but I can't risk getting into it making that assumption.

I'm meeting with the seller tomorrow to discuss, but I want to propose an arrangement where I buy the property at the price point I know matches the justified ARVs, but shares 20% of the profits past a certain price point. 

For example, ARV at the moment is $250k, if I sell for $250k, the seller wouldn't get anything more, but if I'm able to sell for $350k for whatever reason, I would share 20% of that $100,000 difference with them, and they would get an extra $20k on the back end of the deal.

Has anyone ever done something like this successfully? Is there anyway to build it into the Purchase and Sale Agreement, or would it have to be an agreement we sign after closing when I formally take ownership of the property?