Under the direction of Jeff Sutherland (Co-Creator of Scrum) the US Department of Defense has begun to embrace Agile contracts for procurement.
Read a summary of the " Greater Accountability and Faster Delivery Through Modular Contracting "
Their solution is called "modular approach", where the core is incremental delivery and that the specification can be rewritten after each delivery. A typical contract shall contain at least 3 deliveries per year. Read about their procurement guidelines and order " Contracting Guidance to Support Modular Development ".
Thanks to http://agilakontrakt.se for the summary.