Legality of paying a sum to the previous owner when I sell

[John J.]

I'm working on a deal right now, the seller wants 100k and I want to pay 90k.  However the seller has agreed to take the 90k so long as they get a percentage of the profit I make when I sell the property.  We both have agreed to this.

Would this be considered unlicensed real estate activity, whats your take?

[Charlie MacPherson]

@John J. I don't think so, as you're selling a property that you own.  

You should write your contract with a clause that states "Seller is to receive X% of net profits, with the amount not to exceed $XXXXX.

It would be silly to pay him $20,000 in profits to get a $10,000 price reduction.