It can happen that you need a Brigader for a shorter period than the initially agreed schedule. The Brigader set his time aside for the hours you requested and committed to respecting them.
This is why we don’t shorten a mission’s duration once it has been accepted by the Brigader.
We advise you to place your requests for the hours at which you are certain to need someone and then ask a Brigader to work overtime. Very often, they’ll be available!