Master Lease or Seller finance with owner open to both

[Robin Hall]

I'm looking at a small multifamily. The owner is open to creative solutions like seller financing or a master lease with option to buy.  I'm leaning towards seller financing to lock in a rate somewhere around today's currents rates. There are 3 buildings. I would like to leave the mortgage in place and have the owner finance the remaining amount. This sounds tricky for my second multi-family unit. How would I structure this or do I need to explore subject to in respect to the mortgage. I know it's unlikely but the bank could call the loan (which could scare the owner). How to I keep this as simple as possible and present two offers, One master lease option, and one seller financing option.