data_invoice_ferreteria/views/account_invoice_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="account_invoice_data_ferreteria" model="ir.ui.view">
            <field name="name">account.invoice.data.ferreteria</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form" />
            <field name="arch" type="xml">
              <field name="fiscal_position" position="after">
                  <field name="obra" attrs="{'readonly':False}"/>
                  <field name="pedido_por" attrs="{'readonly':False}"/>
              </field>
            </field>
        </record>
    </data>
</openerp>