TOPIC: Procurement Method for Work done in Agile

If I'm understanding it right, Sprints are decide up front based on an iteration that includes a fairly limited and known amount of risk and are small enough to ensure that they are likely to be completed within the time allocated. Nevertheless, I also understand that the first sprints put the stories forward that are the highest risk and with the highest customer value. How then do procurements work? What contract type is appropriate? Time and Materials? Pay per Iteration for a team? Pay per hour per person? I can see that it probably wouldn't matter to the buyer how the iteration gets done as long as it gets done, so hours per team or person wouldn't seem to be sensible. Pay per iteration seems to make sense but not all iterations have equal value, and it would seem that a pay-per-iteration would not achieve a sense of real value until the team performed for 4 or 5 iterations to see its burn-down rate. This all is very very new to me as I've only worked on waterfall projects, so maybe I'm misunderstanding or missing something. Can anyone assist?