When using sales return order, it can happen that the qty to invoice (credit) is different from the returned qty.
Ex : a customer returns 10 pcs but only 6 can be "invoiced" (credited).
As the total qty invoiced is not equal to the quantity to invoice, the sales return order remains open and will never be deleted.
An action should be available to force the "closure" of the order. Ideally the same action should be available for sales and purchase orders