public function refund($transactionId, $payload = '') {
        $res = $this->sendRequest(
            function () use ($transactionId, $payload) {
                $uri = Constants::API_VERSION_ROUTE . '/' . $transactionId . self::REFUND_API;
                $opts = [
                    'headers' => $this->getRequestAuthHeaders($uri, $payload),
                ];
                if (!empty($payload)) {
                    $opts['json'] = $payload;
                }
                return $this->guzzle->post($uri, $opts);
            },
            $transactionId
        );
        if ($res instanceof GenericError) {
            if ($res->errorcode === 1155) {
                $error = $this->module->adminErrorStore->getRefund1155Error();
                $error->transactionId = $transactionId;
                $error->logger->error($error);
            }
        }

        return $res;
    }