Hello Classic School Products,

Edit the order. Click on the "Invoice" button. All backorder quantities will be moved to the shipped quantities. If necessary, edit any items that are still on backorder by changing the ship quantities. Post the Invoice.

Is this what you are looking for?

Or perhaps you have set the "Calculate Backorder" option in the Company profile. If this is set, then Adagio will automatically calculate a backorder quantity by subtracting the qty shipped from the qty ordered.