(aka procurement of agile software development in UK government)
“Fix the price and the scope, and make a contractual agreement to commit to the top 80 percent of the PBIs [Product Backlog Items], ordered according to their value to the client. The vendor might exceed that 80 percent target if it can complete the work within the contracted cost. Offer clients the option of exchanging an emergent requirement for any existing PBI of equal (or lower) value to them, as long as the development cost of the new item is no more than the cost of the original.”
I’ve personally used this approach to good effect (not in the exact detail but close enough) when working as a procurement manager and project manager within UK central government. In the circumstance where the procurement function wasn’t Agile and required a “fixed scope” approach to tendering, but where the development work needed to be done according to the GDS Service Manual.
It satisfied both parties.
Ask any project manager about the key to their success, and they will say that delivering a project is often more like a "dark art" or by chance, than a predictable science.