Owner carry back, but with unique terms

[Kyle Fritz]

Hey everyone!

I am trying to buy two triplexes (must be bought together), and the seller is open to owner-carry back.
The seller would like 20% down, however I can't do more than 10% down.
I would like to get nearer to 5% down if possible, and am trying to structure an offer that she may consider.

What I would like to know, is if anyone would be willing to share an idea with me for how I might be able do this.

And the second question: Is it unheard of to structure a purchase so that the buyer pays the seller a base amount per-month, with a varying extra amount dependent upon the number of occupied units? (example: $20 extra per month per unit occupied). Of course, if this is a terrible idea, that would also be good to know. :)

The units have been on the market for about 2 months. I have not met with the seller yet to know why she is trying to sell, or how quickly she needs to sell.