Question on a sliding scale mortgage / seller financing

[David Lee Hall, III]

I have an opportunity on a property that has a pretty decent range on resale for a flip ($130-$160k). The seller and I are about $16k off because of this. We are open to at least discussing a contingency seller financed portion of the difference. For example, if it sales at the low end of the range, then my acquisition cost is final, but basically for every $10k the sale price increases beyond that, he would get $5k. I don't really have an issue with this, my question would be how to secure this with a second note. (I am trying to get some ideas here prior to contacting my attorney.) Is it possible to have a second mortgage that could potentially be $0 or alternatively $15k depending on a resale price?