No worries, think I misread your requirement. If I've got it right the user selects 'A' and only wants to see the row labels for B + C.
To do this I've created another Filter table (called ExcludeFilterTable ) this has an inactive relationship with the sales table. I've then created a Measure for Sales using the following DAX Expression:-
To construct the Pivot table, place the ExcludedFilterTable into the pivot table filter, place FilterTable into Rows and place Sales(measure created using the above DAX) in values
The value selected in the Pivot table Filter should be excluded from the Row labels.
I've tested this in Excel 2013 Powerpivot and it seems to work well, all be it with a very small data set.
Hopefully this will be a good starting point for you.