Web Intelligence Prompt Question

I have a universe that is build primarily on a sales fact table that has two different revenue fields: transactional amount and local currency amount.
The universe makes these two fields available to reports as measures called Transactional Amount and Local Amount. 

I would like to build a Web Intelligence report that prompts the user on whether they want to report on transactional or local amount, and then returns the correct field based on their prompt selection.  I cannot seem to figure out how to best do this. 

Thanks in advance for any help!