SumIf Lookup Not sure at all

[PFS_ITDept]

My Budget is the sum of sales from a historical sales Date.

In the main sales table the Date column has a corresponding BudgetDate against it so the link is present, just need to "lookup" the two.

I know not correct but simplistically put...

Sum({<[BudgetDate] = [Date]>} [Sales])

[rwunderlich]

I think you need to use Sum(if()) for this case. 

Sum(If(BudgetDate = Date, Sales, 0))

-Rob