cooperative-market/beesdoo_purchase/views/purchase_order.xml

<openerp>
    <data>
        <record model="ir.ui.view" id="beesdoo_purchase_order_form_view">
            <field name="name">beesdoo.purchase.order.form.view</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form" />
            <field name="arch" type="xml">
                <field name="product_id" position="attributes">
                    <attribute name="domain">[('main_seller_id','=', parent.partner_id)]</attribute>
                </field>
                <xpath expr="//page/group/group//field[@name='date_planned']"  position="attributes">
                    <attribute name="readonly">0</attribute>
                </xpath>
            </field>
        </record>
    </data>
</openerp>