Allowing negative quantity in purchases?

It seems we are allowed to create a purchase with negative product quantity. Do not understand the logic of buying negative product quantity from the suppliers. Is there any use case behind this?

From my understanding, it should not allow any negative quantity as we can adjust the over-received quantities by using supplier return shipments or credit memos.

It is to support return to supplier.

If the return is managed by the purchase, it can link the return shipment and the supplier credit note.
And it can go even further as a return line could be added to a standard purchase if it ever happens.

This seems to be the only way to do a return shipment as there is no action to create a return based [in part] on the supplier shipment containing the article(s) to return (as is done quite logically in OpenERP).

Is it possible to create such an action to launch a wizard to create the return move(s) there (where all the pertinent info is available)? Then it could be visible in the returns part of the original shipment.

Also, it seems impossible to enter the suppliers return waybill authorisation number in the reference field on the supplier return shipment form, it’s greyed out(which could be set in the above wizard).

It is patch is welcomed.
But it will require first to implement origin field on purchase line for Issue 4050: It should be possible to include the origin of the return sale/purchase line - Tryton issue tracker

The field is only editable in draft state.

As is currently, seems the shipment return is already in ‘waiting’ state once the return ‘purchase’ is confirmed.
Perhaps shipments should have editable reference fields (as the purchase itself does) even if no longer in draft state in order to enter the correct return id.