Due quantity calculation for ASN

hi,

As per SAP help document there are two types of Due quantity calculation that we can have:

1) ASN based.

2) Confirmation based.

We are having a requirement where in the Due quantity displayed in Delivery tab - Receipt and Requirement - Due list for purchasing documents to display the ASN due quantity for a PO as:

PO due quantity = (PO confirmation quantity) - (PO-ASN quantity)

e.g.

PO request quantity  = 100

PO cofirmed quantity = 80

ASN created for PO with quantity = 20

But the Due list for PO displays the due quantity as 80 which is the confirmed quantity. But is there any way we can show the Due quantity = cofirmed (80) - ASN (20) = 60 in the due list.

thanks in advance,

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