public function refund(array $params)
    {
        // transaction identifier must be set
        if(!isset($params['identifier']))
        {
            throw new MonetivoException('$transaction["identifier"] is not set');
        }
        // other parameters like 'amount' and 'description' are optional

        return $this->merchantApi->call('post', 'transactions/' . $params['identifier'] . '/refund', [
            'form_params' => $params
        ])->toArray();
    }