Ax2009 weighted average - ledger balance vs on hand

Hello everybody,

Can someone help me?

In AX 2009 I have :

1 item with weighted average cost method and I make belows transactions

22/07/2009 - 1 purchase posted invoice with my item qty1 purc price 1

22/07/2009 - 1 purchase posted invoive with my item qty 1 purh price 10

22/07/2009 - 1 P&L journal posted with my item qty -1

I close 07 2009 and everything it is ok

Next month I "realize: that one of invoices are “bad”

so I go on purchase order and I put -1 in receive now field and post invoice with 22/08/2009 posting date,

and second step put +1 in receive now field and post invoice with 22/08/2009, when I post this invoice I change purc price from 1 to 33.

I close 08/2009 and I have diferencies between on hand financil value and general ledger account balances.

Can anybody help?