account_invoice_supplier_view.xml

<?xml version="1.0"?>
<openerp>
    <data>
    <!-- SUPPLIER INVOICES -->
        <record id="invoice_supplier_form" model="ir.ui.view">
            <field name="name">invoice.supplier.form.inherit</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_supplier_form"/>
            <field name="priority">20</field>
            <field name="arch" type="xml">
                <notebook>
                    <page string="Purchase Orders" name="purchase_orders">
                        <field name="purchase_order_ids" invisible="0"/>
                    </page>
                </notebook>
            </field>
        </record>
        <!-- if we do so, you can not create an invoice -->
<!-- 
        <record id="invoice_supplier_restrict_invoice_form" model="ir.ui.view">
            <field name="name">invoice.supplier.form.inherit</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="invoice_supplier_form"/>
            <field name="groups_id" eval="[(6, 0, [ref('group_restrict_edit_invoice')])]"/>
            <field name="priority">20</field>
            <field name="arch" type="xml">
                <field name="invoice_line_ids" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','in_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </field>
                <button name="%(account_invoice_prices_update.action_update_prices_list)d" position="attributes">
                    <attribute name="invisible">True</attribute>
                </button>
                <xpath expr="//page[2]//field[@name='company_id']" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </xpath>
                <xpath expr="//page[2]//field[@name='account_id']" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </xpath>
                <field name="origin" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </field>
                <field name="date_invoice" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </field>
                <field name="partner_id" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </field>
                <field name="user_id" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </field>
                <field name="fiscal_position_id" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </field>
                <field name="date_due" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </field>
                <field name="payment_term_id" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </field>
                <field name="partner_bank_id" position="attributes">
                    <attribute name="attrs">{'readonly':[('type','=','out_invoice'),('purchase_order_ids','!=',[])]}</attribute>
                </field>
            </field>
        </record>
         -->
    </data>
</odoo>