You can use partial sums (AKA subtotals / totals, which you can enable in presentation tab per dimension) and use dimensionality() resp. secondarydimensionality() to switch your one expression to show your discount percentages in your detail columns and the sum in your total column.
Something along these lines:
=if(SecondaryDimensionality() = 0,
Excellent answer swuehl! That worked perfectly. Any suggestions on how to format the two ( sales, percent ) seperately? I currently have my expression formated as "Fixed to" 1 decimal place and "Show in Percent%" which is fine for the percentages but not the sales.
Here's my expression now:
If(SecondaryDimensionality() = 0, If(Sum([Discount Percent]) > 0, Money(Sum([Sales Amount]))), [Discount Percent])