Pivoting with NULL Dates
So at this point in the code:

isnull((processeddate),' ') as 'Null'


The column processeddate is not visible to SQL Server, because you are selecting available fields from the result of the following query:

SELECT EmpID, EmpDeductAmt, month(ProcessedDate) as TMonth
  FROM [dbo].[Invoices] where Inv_Number not in ('1035','1039')


...along with the pivoted columns [1] through [12].

You either need to include ProcessedDate in the above query, or exclude it from the outer SELECT.