After Deleting delivery the Qty still remains reserved

Once you delete a delivery in sap ,the qty of the delivery item should be transformed into open qty for the future dellviery.But this is not happening as on sale order in the schedule line in tab it still shows "delivered items like 40" altough the delivery has been deleted.

The consequences are major as never a delivery of that item can be created as there are no open qtys and that stock cannot be used as well.