There are a couple of ways to do this. The first is to put a table calculation on the measure. Right click on the measure on the view, select Quick Table Calculation-->Difference. If it's not already, right click on it again and select Compute Using-->Table (Across).
A second option is to create a calculated field for each year and set it to the measure. Then, subtract the two from each other. Similar to the following:
if [Fiscal Year]='FY2011' then Sales end
If [Fiscal Year]='FY2012' then Sales end
The resulting calculation would look like the following:
sum([FY2012 Calc])-sum([FY2011 Calc)
Using the latter option, the FY field does not need to be on the view.
Hope this helps!