public function refund($billId, $refundId, $amount='0', $currency='RUB')
    {
        return $this->requestBuilder(
            $billId.'/refunds/'.$refundId,
            self::PUT,
            [
                'amount' => [
                    'currency' => (string) $currency,
                    'value'    => $this->normalizeAmount($amount),
                ],
            ]
        );

    }