In some industries, customers commonly place orders for a certain volume of product and ask for regular deliveries from an order up to the total amount on it. This principle, called open orders, is managed by the sale_delivery module in OpenERP.
OpenERP handles open orders easily. An open order is an order for a certain quantity of products but whose deliveries are planned for various dates over a period of time.
To do that you must install the sale_delivery module (in addons-extra at the time of writing). A Sales Order is entered as a normal order but you also set the total quantity that will be delivered on each order line.
Then you can use the new tab Deliveries on the order to plan the quantities sold and enter your delivery planning there.
In the order lines, OpenERP shows you the quantity planned in addition to the quantity sold so you can verify that the quantities sold equal the quantities to be delivered. When you confirm the order, OpenERP no longer generates a single delivery order but plans scheduled despatches.
If you work with Open Orders, you should set the order into the mode Invoice from the Packing. Then the storesperson will be able to replan and change the quantities of the forecast deliveries in the system.