From what I see, it’s very simple. But then again, I am no expert my any means.
From what I am gathering, if your agreement is assignable, then your new buyer is the one who qualifies for the loan, not you (you aren’t buying the home, after all). The owner you leased optioned the house from will get a check from escrow for his asking price, and you will get a check for the difference between the original asking price and the new loan purchased price.