Problem with 'C' transaction

Hi,

We are on 6.1 MM.  I try to enter a 'C' transaction of -ve qty through INV500 
to reverse another 'C' transaction generated by vendor invoice posting.  
However, a message comes up saying that the warehouse is invalid, which is 
actually not prompted for input.  The definition of transaction effect 'C' is 
as follows:

Transaction Type    C
Description           P.O. INVOICE RECEIPTS
Ref No./ Description  P.O. NUMBER   Affect Resupply order
Location Description  LOCATION      Check Resupply order
Cost Description      ACT TOT COST  Affect PO Qty in Inspection
Comment Description   INITIALS      Redesignate Transaction
Opening Balance                     Affect Actual Cost    Y Affect Y/not Y
Receipts                  Warehouse Level
Adjustments               Lot Master                        CONTAINER
Issues                    Cost Adjustment      Y
Sales Units               Cost/Price Entry     Y   Trigger QA Activity
Sales Amount              Multiple Issue
Item Last Tran Date       Check Purch Order    Y   Spread Receipt
Whse Last Tran Date       Check Shop Order
Affect Shop Order         Check Purch U/M      Y
Aff PO Quantity           Location Level
Aff PO Cost           Y   Shop Order Issue
                          Affect Cycle Count     Program/Procedure

I try to clear the Location Description to see if it's the problem. But the 
Warehouse Level is then set to Y automatically.  Putting a value back to the 
description field even force me to put a Y to Location Level, which then again 
set Warehouse Level to Y automatically.  If Warehouse Level and Location Level 
are set to Y, what will be affected?  The help says that the Quantity field in 
the warehouse master and lot / location master will be updated.  But I can't 
find any quantity field in these masters.  I thought this transaction effect 
was used to update actual cost only, why is warehouse involved?

Does anyone has a clue on what's going on?  Thanks in advance for any 
suggestion.

Eric Tam