public function invoicePartAmount($invNo)
    {
        $this->_checkInvNo($invNo);
        $this->_checkArtNos($this->artNos);

        //function activate_part_digest
        $string = $this->_eid . ":" . $invNo . ":";
        foreach ($this->artNos as $artNo) {
            $string .= $artNo["artno"] . ":". $artNo["qty"] . ":";
        }
        $digestSecret = self::digest($string . $this->_secret);
        //end activate_part_digest

        $paramList = array(
            $this->_eid,
            $invNo,
            $this->artNos,
            $digestSecret
        );
        $this->artNos = array();

        self::printDebug('invoice_part_amount', $paramList);

        $result = $this->xmlrpc_call('invoice_part_amount', $paramList);

        return ($result / 100);
    }