How to slightly extend borrowing power?

[Kyler J Sloan]

I have a neighbor that wants to sell their place to me and I want to buy it. 

However, the current interest rates are killing my borrowing power.

Given that we are mutually interested in coming to an agreement, besides a cosigner, what strategies would you recommend for me locking it in with him prior to saving my full down payment amount? 

i.e. sellers financing 1 / 3 to balloon later and conventionally financing 2  / 3?

Thanks in advance for any insight