How to remove a not completed purchase order?

I am a IT person working in a Germany compnay at Beijing China, we are using Navison 3.1 and we have a problem to remove a NOT completed Purchase Order. The problem is: A original purchase order quantity is 100Kg for example, but actual received and invoiced quantity is 80Kg, the rest 20Kg will never be delivered again. So this PO will not be finished and would be kept by system for ever. How can I convert this kind of PO into Posted Purchse Order list? For Sales Order, same issues may happened also I think. Thanks in advance!

Reopen the PO, Set quantity to 80, Delete the PO.

Thanks for the help! I tested, I could delete the Purchase Order whether I set Quantity to 80Kg or just keep the original 100Kg, does it make any sense to set purchase quantity to received quantity (80Kg)? Thank you very much!

Just delete the order. If you want to, then create an archive before you delete it, but you are correct in your assumption that thtere is no value in changing the Order Quantity unless you have a business requirement to do so.

Thanks everybody who give me advice! Thanks a lot!