Simple Calculation within pivot table
 

I am trying to determine the difference in our inventory balance from month
to month. My pivot table currently looks like

Date
Buyer Part # 8/31/06 9/30/06
TC ABC $60 $50
TC DEF $100 $100
Subtotal $160 $150
KM GHI $20 $30
Subtotal $20 $30

I have used calculated item - the formula is 9/30/06 - 8/31/06. The problem
I am experiencing is that the formula will add part #GHI under buyer TC and
buyer KM will get the 2 part #s that belong to buyer TC in addition to the
one part she is responsible for. The values are zeros and the calc is
working, but the pivot table becomes enormous. Every buyer gets every part #
listed under his/her name - how do you only find the difference is the
inventory value for parts that belong to the specific buyer?