<button name="action_invoice_sent" type="object" string="Send by Email" attrs="{'invisible':['|',('sent','=',True), ('state', '!=', 'open')]}" class="oe_highlight" groups="base.group_user"/>
<button name="invoice_print" string="Print" type="object" attrs="{'invisible':['|',('sent','=',True), ('state', '!=', 'open')]}" class="oe_highlight" groups="base.group_user"/>
<button name="action_invoice_sent" type="object" string="Send by Email" attrs="{'invisible':['|',('sent','=',False), ('state', '!=', 'open')]}" groups="base.group_user"/>
Eliminando lo que está en color, el botón reaparece ....