If you are trying to build a calculated member, you just have to write the expression as

([Measures].[Sales],[Good Dispatched] - [Measures].[Sales],[Goods returned])

(Please add appropriate expression for the dimension)

Build the cube and...

Instead of using where clause you can provide an Expression in the Table column like

=iif(Parameters!Param.Label="First",

Fields!Reseller_Extended_Amount.Value,

iif(Parameters!Param.Label="Second",Fields!Reseller_Gross_Profit.Value,

Fields!Reseller_Freight_Cost.Value))

assuming that Reseller_Extended_Amount,Reseller_Gross_Profit.Value and Fields!Reseller_Freight_Cost are 3 measure u would like to view.

Depending on the Parameter...