I'm not experienced with Cross Tabs, but they appear to be mostly to do with formatting so I think this will work.
You could try creating a Formula to do the subtraction of one table from another.
In the Field Explorer, right click "Formula Fields", select "New", name it "Sales-Return".
Have one line of code similar to:
{tablename.sales} - {tablename.return}
Click OK, and then use this Formula as part of your Cross Tab setup.
Edited by Erik - 18 May 2015 at 7:32am