After creating a budget, its financials will need to be set up in a specific way to mirror how a lump sum budget would work.
First, create a Fixed service with 'Piece' set up as a Unit that will reflect the total amount of the budget, and set its quantity to 1.
Make sure that both expense and time tracking options are disabled so people cannot track either of those on that service. By using this method, you are basically 'locking' this service:
Underneath it, you will now be adding Fixed services against which people will be able to track time. This time, make sure to enable the time tracking option but set the quantity to 0 so no extra budget will be added to the initial value:
Then, after people start tracking time, those services will 'use up' the total budget and you can see at any point in time how much budget is left: