This is a common problem but there is a way around it using an advanced formula. Try something like:
IF FIRST()==0 THEN WINDOW_SUM(SUM([REVENUE]),0,IIF(FIRST()==0,LAST(),0)) END
I think you need to partition this by OpportunityID to make it work - in other words put OppId on the Level Of Detail shelf and set the Compute Using of the advanced table calcs.
If you're unfamiliar with table calcs you'll probably find this challenging to get right - this link contains a comprehensive guide to table calcs: http://drawingwithnumbers.artisart.org/want-to-learn-table-calculations/