Yes, imagine that you're over budget for the month: -$100.

If you have a category with a positive balance of $45, clicking on the positive category balance should pop up a dialog that asks if you want to cover overspending (overbudget?). Selecting that will subtract as much as needed, or as much as possible to cover the negative up top. So, in this case the category would drop to $0 and available to budget becomes -$55.

If you have a category that has $324 in it and use it to cover overspending (overbudget?), then the category drops to $224, and available to budget becomes $0.