In an Back to Back Order Sales Order , we can update the quantity on sales order line even when a Approve Purchase order exists in Purchasing . As for documentation this is a valid behaviour.
In some case we have build customization to Inform a particular user about updatation by sending workflow notifications.
There is new functionality starting R12 that if you make changes to SO line quantity it will automatically update the PO quantity for that B2B order. Please check R12 product doc for more details and exceptions.ReplyDelete