If in an event where only a few Products needs to be returned to your Supplier, Product's Cost will also need to be returned exactly as per the original Purchase Order.
At the Supplier Return screen, remove the unwanted Products by simply deleting them from the list.
At the time they have been deleted, your Cost Lines will still remain unchanged, therefore this data will need to be refreshed to reflect the correct remaining Return Cost. Until then, the Supplier Return will continue to fail to Complete.
Simply click the circled arrow button to refresh the Cost, until the Total Return Cost is equal to Total Cost Line.
Once these two costs are matching, the Supplier Return can now be safely Completed.