I have read many posts about PO Limit process, explaining account assignment U (unknown) can be used with item category B (limit). But none explain on how to do the invoice receipt for the item with unknown assignment.
If I use cost center (K) in the PO Limit, I can input the amount in the account assignment popup dialog. The G/L account and the cost center adopted directly from PO. But for unknown, I can only input the amount but cannot do anything to change the account assignment, thus failing to complete the invoice process. I have tried using the account assignment layout. I have the screenshot for example.

Can anyone elaborate on how to do this? Thank you.