Middleman on Sale of Property

[EDWARD M.]

For purposes of simplifying, I will use round numbers.

I have a seller (family member) looking for $100,000 for his property. I have an investor that is willing to pay $120,000 for the property. I was looking to see if there was any legal agreement in which the seller agreed to pay me anything in excess of the $100,000 paid for by the buyer (anything besides me setting up a wholesaling agreement). Any advice? How can I set up this transaction?