Non-billable work within a work item/budget

UseCase

• An offer for a feature is mapped in a DevOps feature and estimated, offered and commissioned with a number of hours.
• The estimation is done for the whole feature, but only certain work is billable (for whatever reason). These can be work items of the feature, or also certain activity types in any work item. (In other constellations, this may also apply to certain iterations, but I can't think of an example myself).

Expectation

• When creating the budget, I would like to be able to define which times are billable / not billable, i.e. excluded from the budget. From my point of view, the means of choice would be to mark the activity type with the option "Non-billable".
• For both types (work item and/or activity type), I would like to see in a comprehensible way which times fall / have fallen out of the calculation.