Calculation Field problem

Hello All

Before i start i just want to say sorry if my question don't make a lot of sense please ask for more info if you need.

I have a workbook with 2 sheets in it sheet one is called goods in with 9 standered fields for products

Date ;PO no ; Code ; Description ;Size ; Qty ; Qty left ;Location ;Notes

I time stamped my date column.

My second sheet is called Products out

Date ; PO no ;Code ;Description ;Size ;Order No ; Qty Out ;Notes

Date is Time Stamped PO no is doing a look up to goods in for the relevant PO no so is code; description; size from the goods in sheet

This is where i im stuck what i am trying to do is take the Qty out and deduct it from the Qty in to give me the qty left for specific PO no and Product Code is there a way i can do it in excel

I have some VB knowledge but not enough to safe my life im using Excel 2007

thanks in advance for all help and assistance.

No Problem i do have the knack to confuse even my self most of the time but ill try and put it down as i see it in my head.

Basically it works like a warehouse program

1: Goods come in on a PO (lets say 100 come in on PO1234)
2: i put it in a location A-1-B
3: i get an order (CSG0001) that is Linked to the PO1234 but they only want 50 (that leaves 50 still in location A-1-B)
4: a month later i get and order (CSG01001) when i type in the PO1234 in and it brings up that line it should show that i have 50 left and not the 100 that came in originally.

That is basically what i am trying to accomplish with my limited knowledge.