You can't easily use set analysis expressions in this case. The sets are calculated per chart, not per row. So you get one set for all the stores instead of a set per store like you want. You can use if statements instead. Something like:
Sum( if (POSDate >= $(vMinSIRDate) and POSDATE <= max(RunDate),SalesDol))
Great, thank you Gysbert!
I had to make some slight modifications, but that was the answer I was looking for.
Sum(If(POSDate >= Date($(vMinSIRDate)) and POSDate <= Date(RunDate),Aggr(Max(SalesDol),ItemKey,POSDate)))
Without the aggregation and max on the sales dollars, I was getting extremely high numbers--multiple rows were being summed for each ItemKey/POSDate.