Jeff wrote excellent blog about applying Scrum into sales process.
I get this question about how to do fixed price & scope subcontracting over and over again and I usually start asking about their partnership with the customer. In most cases it is all about trust (or politics..) ... actually lack of it. Contracts are made to cover up the areas where there is no trust between the parties. I am not saying that you do not need contracts - you do as we are doing business after all - but I think the focus should be more on collaboration and communication about incentives rather than what are the delivered features and what is the time frame (and what are the sanctions if supplier won't make it).
As Mary Poppendieck has done excellent job putting this all down to a presentation I won't elaborate it more here for now.
Totally about different topic, but still important enough: It's often surprising how incentives end up pulling people in the wrong direction. See daily WTF for real life example.