Arjun,

We can't help without a Tableau Packaged Workbook which would allow us to replicate.

I agree with Tom - a workbook would be helpful. With that said, ATTR() returns * when there are multiple dates, whereas MIN returns the minimum, so the formula in your post should return null whenever you have multiple dates for a category breakout. You are probably trying to do something more like

SUM(IF [Date] = {FIXED : MIN([Date])} THEN [Sales] END)