factura_venta_tomodent/res_currency_view.xml

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

<openerp>
    <data>
        <record id="res_currency_en_letras" model="ir.ui.view">
            <field name="name">res.currency.en.letras</field>
            <field name="model">res.currency</field>
            <field name="inherit_id" ref="base.view_currency_form" />
            <field name="arch" type="xml">
                <field name="symbol" position="after">
                    <field name="en_letras"/>
                </field>
            </field>
        </record>
    </data>
</openerp>