ticket_venta_posprint/views/invoice_button.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
  <data>
    <record id="view_invoice_form_ticket_posprint" model="ir.ui.view">
      <field name="name">account.invoice.form.ticket.posprint</field>
      <field name="model">account.invoice</field>
      <field name="inherit_id" ref="account.invoice_form"/>
      <field name="arch" type="xml">
        <xpath expr="//field[@name='number']" position="after">
          <button name="action_print_ticket_direct"
                  type="object"
                  string="Imprimir Ticket"
                  class="oe_button"/>
        </xpath>
      </field>
    </record>
  </data>
</openerp>