New roof before closing

[Justin Foster]

I have this house under contract with a friend of mine and it needs a new roof before we close - as is its uninsurable and we live in oklahoma. He can't pay for it so we agreed to knock off 10k from the price and I'll pay for it. So I'll be shelling out a bunch of money on a roof I don't quite own yet. How do I best protect myself in this? He said if the deal failed to go through, he would pay me back within a year. Is this a side contract or a contingency that can be written into the contract? Should there be another contract of some sort that says I can put a lien on the property for the value of they roof if I buy a new roof and the purchase somehow doesn't go through. I would plan on trying to schedule the roof replacement as close to closing as possible to limit some of the risk. I'm new to all this so any advice is appreciated. Thanks!