Has anyone done a partial purchase option agreement?

I am currently putting together a contract for a partial purchase option agreement. The reason its a partial is that I only want to buy the portion of the property that is over and above the portion that the seller currently has a mortgage on.

I’m doing this as I have the contract set such that the seller will quit claim me to title once I have an end buyer and then I will sell the property and be a co-owner with the current seller.

The only reason I’m considering doing it this way is so that when I get quit claimed to title the amount of excise tax that will need to be paid will be 20-25% of what it would be if I just did a straight purchase option agreement.

I can see the flaw in this in that the existing seller will be involved as a seller, but I plan on nailing that persons total take at the time of signing the option agreement.

Please someone shoot some holes in this for me. It shouldnt be hard to find flaws, I just want plenty of other opinions on this. Oh and I know that I’m really doing this as a retail deal, and this is the wholesale forum, but didnt know where else to ask this one?

Thanks for your help.