How can I allow negative quantities (product_qty) to be entered on a Purchase Order Line. Now it shows a red line and I cannot save the form.
I'd like to add negative quantities to the PO in order the return goods to the vendor and also create a Delivery (outgoing stock picking) from these.

In this view it is not possible to add new products (only product already existing in the delivery). I need this functionality to add new products so I can return empty goods to the vendor on delivery
You can only return something you have received. If you need to send something to a vendor you have not received, you must sell it. Use a sales order and send an invoice.
Vendor might send you a Credit note for that sales order.