For a top down budget, you may want to consider using one of the custom cost
fields. You could assign a budget number to that task in the Cost1 field for
instance. Howerver, this value will not change (such as Remaining Budget)
unless formulas (or other code) is generated to handle the calculation.
Generally, it is best to use project to keep track of the allocated budget
and then maintain any other reserves, etc. separately. You could, however,
establish some tasks that represent management reserve or undistributed
budget. Be careful, since most programs distribute budge by WBS (Work
Breakdown Structure) and you cannot violate that business rule when moving
money around.
Project is for scheduling, other programs are for budgetting.