account_timbrado/views/account_view.xml

<?xml version="1.0"?>
<openerp>
    <data>
        <record id="view_account_journal_form" model="ir.ui.view">
            <field name="name">account.journal.form</field>
            <field name="model">account.journal</field>
            <field name="inherit_id" ref="account.view_account_journal_form"/>
            <field name="arch" type="xml">
                <field name="company_id" position="after">
                    <field name="timbrado" attrs=" {'required':[('type','=','sale')]}"/>
                </field>                
            </field>
        </record>
        <record id="view_account_invoice_form" model="ir.ui.view">
            <field name="name">account.invoice.form</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_supplier_form"/>
            <field name="arch" type="xml">
                <field name="supplier_invoice_number" position="before">
                    <field name="timbrado"/>
                </field>                
            </field>
        </record>   
    </data>
</openerp>