Making a running total

I've got the following query:

SELECT     TOP (100) PERCENT dbo.OPOR.DocNum, 'Inkooporder' AS DocType, dbo.OPOR.CardCode, dbo.OPOR.CardName, dbo.POR1.OpenQty AS [Te Ontvangen], NULL AS [Te Leveren], Convert(DateTime, CONVERT(VarChar(32),dbo.POR1.ShipDate))
FROM         dbo.OPOR INNER JOIN
                     dbo.POR1 ON dbo.OPOR.DocEntry = dbo.POR1.DocEntry
WHERE     (dbo.POR1.OpenQty > 0) AND dbo.POR1.ItemCode = '6004005'

UNION ALL

SELECT     TOP (100) PERCENT dbo.ORDR.DocNum, 'Verkooporder' AS DocType, dbo.ORDR.CardCode, dbo.ORDR.CardName, NULL AS [Te Ontvangen],  dbo.RDR1.OpenQty AS [Te Leveren], Convert(DateTime, CONVERT(VarChar(32),dbo.RDR1.ShipDate))

FROM         dbo.ORDR INNER JOIN
                      dbo.RDR1 ON dbo.ORDR.DocEntry = dbo.RDR1.DocEntry
WHERE     (dbo.RDR1.OpenQty > 0) AND dbo.RDR1.ItemCode = '6004005'

The result is an overview of item deliveries and items to deliver. Now I would like to have a running total that will take the on hand quantity (OITM.Onhand) as base and then decrease that amount with dbo.RDR1.OpenQty AS [Te Leveren] and increase that amount  with  dbo.POR1.OpenQty AS [Te Ontvangen]. How should the query be?