factura_venta_gavilan/account_invoice_view.xml

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

<openerp>
    <data>
        <record id="sale_order_cred_cont" model="ir.ui.view">
            <field name="name">sale.order.cred.cont</field>
            <field name="model">sale.order</field>
            <field name="inherit_id" ref="sale.view_order_form" />
            <field name="arch" type="xml">
                <field name="fiscal_position" position="after">
                        <field name="contado"/>
                        <field name="credito"/>
                </field>
            </field>
        </record>
    </data>
</openerp>