Amending On Order Quantities in Purchase Orders

SUGGESTED

I have a PO which is a part order

We ordered qty 180 units and have GRN 159.96.  The remaining balance will not be delivered.

I have tried to amend the order qty to 159.96 but get an error saying the qty is less than the delivered plus on order

How can I reduce this down so it completes?

The only methods I found searching Sage Help were to delete the PO (thanks, Sage - the whole thing deleted) and take it off order - but that shows it as status Cancel rather than the true status which is complete