inventory valuation

We have following situation:

When the PO is created the Freaight cost is unknown. We are using Moving Average for inventory.After the GR and invoice receipt have been completed, we receive, additional freight invoice. Based on this invoice, by using, credit/debit memo functionality we can assign the cost to the inventory of the article that is still in DC. Meaning, original PO had qty of 100, but by the time the freight invoice came in some of the original qty of 100 is ( ie 80) is gone from DC. The freght cost would be divided over the remaining qty of 20.

The question is what happens when the freight invoice shows up after all of oroginal qty of 100 is gone? Where does this cost get assigned to? Shouls there be some kind of G/L account set up to handle this? and if there was an account how does this cost get assigned to article later>

Need help.

Thanks

Raj