invoice_view.xml

<openerp>
  <data>
        <record id="invoice_form_print_cancel" model="ir.ui.view">
            <field name="name">account.invoice.form.print.cancel</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form" />
            <field name="arch" type="xml">
                <button name="invoice_print" attrs="{'invisible':['|',('sent','=',True), ('state', '!=', 'open')]}" position="attributes">
                    <attribute name="attrs">{'invisible':['|',('sent','=',True), ('state', 'not in', ['open','draft'])]}</attribute>
                </button>
                <button name="invoice_print" attrs="{'invisible':['|',('sent','=',False), ('state', '!=', 'open')]}" position="attributes">
                    <attribute name="attrs">{'invisible':['|',('sent','=',False), ('state', 'not in', ['open','draft'])]}</attribute>
                </button>
            </field>
        </record>

  </data>
</openerp>