Seller Financing + Bank Financing Question

[Landon Eskew]

Looking for some hopefully simple advice on a eureka moment I just had.

I've got a house that a seller is willing to hold financing on as it is owned outright. 4800sf house, 3 car garage, built in 2013, comes with 100 acres of land and an 8 acre water pond. We would be looking to make this our forever home. They've offered the first round of terms as:   Price $699,000 30% down 10% interest Amortized 30 years with 24 month term balloon
Except I don't have $210K handy yet, but the full amount will be available within the next year so I countered with this instead:
Price $699,000 (pending some small conditions) A non-refundable 5% down ($35k) to lock it in Multiple graduated balloon payments of * $100,000 additional due by 5/15/19 * $250,000 additional due by 10/1/19 10% interest Amortized 30 years with an 18 month term balloon No prepayment penalties Buyer keeps property taxes and insurance paid and current Seller chooses closing company to facilitate the transaction
I sent that off two days ago and haven't heard a peep back yet and I'm starting to get anxious... So, my bright idea is that why can't I get a bank loan for 210K and have him carry the rest? Then I have the full down payment and I'm ready to go.
Except the bank will want to have first position while the seller gets second position and that will probably turn them off. Any ideas on how to go about things creatively so I can make this work?