roof repair and warranty - how to work this

[Terry Royce]

So, my girlfriend is buying a rehab property. We discovered during the inspection process that the roof when replaced was not flashed properly and thus leaks by both the chimneys. All the roofers that come out only want to replace it and not repair it, because they don't want to "ruin their name" by repairing it and it leaking again. And replacement quotes have come in from 9k for the low quote to 25k for the high quote.

Anyway the seller has a roofer who agreed to do the repair work on the side for 2,400 and we would split the cost. THe problem is obviously there is no warranty with repair work, but the seller agreed to warranty the work for 2 years. How do we get a "warranty" would a simple contract of sorts work?
Also, should we have the work completed before settlement?

What's the best way to work this.

Thanks
-Terry