item charge and credit memos

gri

hi
i have the following issue:

1. purchase invoice for 10 pieces
2. credit note for 3 pieces
3. item charge invoice for the 7 remaining pieces.

problem:
when i try to apply the item charge invoice, i cannot assign the value to the 7 pieces. i have only the option of assigning to the 10 pieces from the purchase invoice and the adjust cost will make entries also for the credit note.

does anybody has any idea about how should i apply the item charge in order to increase the value of the 7 pieces existing in the inventory with the full amount of the item charge invoice?

thanx in advance, looking forward for your replies.