From In-Portal User Manual

Items

This tab lists all items in the order. For incomplete and new orders, the administrator can add, edit and delete items on this list. For each item, the two modifiable fields are Quantity and Price. The recalculate button in the toolbar updates the order total.
Using this functionality, the administrator can set any price for any product for new and incomplete products. However, the quantity will still be validated – for items with enabled inventory control, even the administrator will not be able to include more items that available (the same backorder logic as on the front end will apply).