Sum Beginning Inventory in Group Footer

I am trying to format a report that will show beginning inventory and determine ending inventory based upon transactions.  This seems to be a simple matter. However, I keep coming up with the wrong answer.
There are four tables (keys denoted by *)
Product:                 *ProductCode / Description
Warehouse:            *WarehouseCode / Description
ProductWarehouse: *ProductCode / *WarehouseCode / Beginning Inventory
Transactions:          *ProductCode / *WarehouseCode / *TransactionID / TransactionDate/ TransactionAmount

Report is formatted in the following manner
ProductCode ProductDescription
       WarehouseCode WarehouseDescription                   Beginning Inventory: 2,500    Ending Inventory 1900 (BeginInv - Sum(TransactionAmt))
        11/15/2006    100                                                                                                                       (This works)
        11/16/2006    200
        11/17/2006    300
        Total             600
 
The problem occurs here. If I have more than one warehouse within a specific product code, I cannot sum the Beginning Inventory and subtract the sum of the transaction amounts (=sum(BeginInventory) - sum(TransactionAmount).  In the case listed above there are three detail records. Access adds 2500 for each detail record. Thus my equation becomes 7,500 - 600, not 2,500 - 600. It is adding 2,500 for each record. I hope I have made this clear enough for someone to assist.