If there are no purchase orders, you can only make subsequent changes to quantity or delivery data in the sales order. These changes are always automatically copied to the corresponding purchase requisition, providing this is permitted by the release status.
If a purchase order already exists for the business transaction and you make changes in the sales order, you must also make these changes manually in the purchase order as it is not updated automatically. Alternately, you can change the quantities and dates in the purchase order and the system will then copy them automatically into the sales order.
See:Changing Sales Orders