You can sort that way, but only within each grouping; the subtotals of that grouping (whether you can see the subtotals or not) are sorted, so you need to do the following for all the fields (columns). [Don't expect the entire table to be sorted by sum of values, that would ungroup the other groupings! The only way to do that is by adding a Net Amount column as the leftmost column of the pivot and sort on that.]:
then: