pos/pos_order_picking_link/views/stock_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>

        <!-- stock.picking -->
        <record id="view_picking_form" model="ir.ui.view">
            <field name="name">stock_picking_invoice_link.stock.picking.form</field>
            <field name="model">stock.picking</field>
            <field name="inherit_id" ref="stock_account.view_picking_inherit_form2"/>
            <field name="arch" type="xml">
                <field name="invoice_state"  position="after">
                    <field name="pos_order_ids" widget="many2one"/>
                </field>
            </field>
        </record>

    </data>
</openerp>