This can be done with parameters and custom calculations, see the workbook below.
The idea is to create a custom table calculation for % (just do a 'quick table calculation' on Sales and save it via 'Customize...') and create a parameter to allow user to pick Sales or %. Then create another calculation that will read the parameter and show either sales or %. You can download the workbook and dissect it to see how it works.
The problem is that there is no easy way to get % to display properly with this approach, i.e. the label will show 0.16 instead of 16%.