Hello,
you Can achieve this adding a ROW_NUMBER() function my SQL query
IN SQL :
SELECT PromotionName,PromotionAmount, Spent, ROW_NUMBER() OVER (PARTITION BY PromotionName ORDER BY PromotionName ASC) AS Rank FROM Table
IN SSRS :
=SUM(IIF(Rank = 1, PromotionAmount.Value, 0))