It can happen that the total returned quantity does not need to be credited.
For example, based on a sales order with qty 100, the customer returns the whole qty of 100 pcs but after analysis, only 60 have to be credited to the customer, the other 40 will never be credited for any reason.
As the qty invoiced (credited) will never be 100 but only 60, the sales return order will never be deleted and will remain open.
An action should be available in order to force the closure of the return order. ideally, the same action should be available for sales and purchase orders.