I am not sure, if you can get sales and return in one row (as 2 columns)
An alternative is to let it as it is, with different records for sales and return.
In the Report, you can probably make 2 grids (next to each other, one with filter Qty>0 and the other with Qty<0, or you can even add a sort field on Qty. eitherway, it Looks like a Group.