Sold before I'm finished!

[Micah Livengood]

My father and I are currently working on a flip, I got a call today from a prospective buyer saying that he is pre approved from the bank and is wanting to buy the house when we are complete. His lease doesn't end until May 2016. At what point do I get him to sign a purchase agreement? He has expressed to me that this is where he wants to live. He knows that he will have a great house when we are finished. We have about 50% of the work complete. This flip is all out of our pocket. We are financing it through our masonry company.