Purpose of the “Appl.-from Item Entry" field?

(Using NAV4SP2)
I’m having some problems with Credit Memos and the closing of Item Ledger Entries.
The problem can be reproduced quite simple like this in the Cronus company:

1) Create a Purchase Credit Memo for 2 pieces of Item No. 2200 and Post it.

2) Create a Sales Credit Memo for 2 pieces of Item No. 2200, add the “Appl.-from Item Entry” column, and lookup and pick the open Item Ledger Entry created before.

3) Now post the Sales Credit Memo.

4) Go to the Item Card of 2200 and look at the Item Ledger Entries.

Now I would expect them to be closed, but they are both open. Why???

If you try is again for a different item (also without existing Item Ledger Entries) but without entering the “Appl.-from Item Entry” it works fine.

Of course you could say I should simply tell my users not to fill this field, but we really need to have the sales entries applied to the exact purchase entries (long story) and we need to post the Purchase Credit Memo before the Sales Credit Memo (even longer story).

I have been doing some debugging and ended in codeunit 22, where I found two problems.
One is the Item Application Entry table, which I don’t know anything about. :-( Anyone??
The other one is this condition in ApplyItemLedgEntry which simply rejects the entry I want it to use:
…
IF TempCostApplnChain.GET(ItemLedgEntry2."Entry No.") OR
(ItemLedgEntry2."Entry No." = ItemJnlLine."Applies-from Entry")
THEN <try another entry>
…

Yes, I have searched both MS Knowledgebase (and found nothing) and Mibuso (and found several posts that seem to find this strange but “by design”) – haven’t anyone an explanation or even a solution or just some ideas to solve it?

Thanks in advance.