Subcontract PO item withdrawal issue when alternative BOM item used

Hi,

As title mentioned, when we issue alternative material for a subcontract PO in BOM, but it still shows the quantity from primary item.

(For example, we have a product, let's say it as Product-A, it's combined by material-B1 and Material-C1

And the BOM for Product-A for item material-B1 has an alternative item, material-B2

Now the issue is when we issue material-B2 instead B1, but the PO usage/withdrawal listed in B1)

Because we are making a list for China custom for material usage, the issue listed above makes our list not available for custom department.

Please let us know how we could make the withdrawal in PO shows B2.

thanks,

Steven