In a traditional approach, the fixed part is the Scope and the other factors such as Time and Cost is changed (of course by raising change requests) as per the changes in Scope.
Agile Methodology as defined by DSDM, fixes cost, quality and time at the outset and uses the MOSCOW prioritization of scope into musts, shoulds, coulds and won’t haves to adjust the project deliverable to meet the stated time constraint. The non-negotiable part is the Time and Cost. That is, you need to deliver features in a fixed time and at a fixed cost. What to deliver can be adjusted as per prioritization and as per the estimates in a given time and given cost.