ticket_venta/views/invoice_view.xml

<?xml version="1.0"?>
<openerp>
  <data>
    <record id="view_invoice_form_ticket" model="ir.ui.view">
      <field name="name">account.invoice.ticket.button</field>
      <field name="model">account.invoice</field>
      <field name="inherit_id" ref="account.invoice_form"/>
      <field name="arch" type="xml">
        <xpath expr="//header" position="inside">
          <button name="print_ticket_ferreteria"
                  type="object"
                  string="Imprimir Ticket"
                  class="oe_highlight"
                  attrs="{'invisible': [('state','=','draft')]}"/>
        </xpath>
      </field>
    </record>
  </data>
</openerp>