public function refundAmount($amount_in_cents, $refund_method = 'credit_first') {
    $doc = $this->createDocument();

    $root = $doc->appendChild($doc->createElement($this->getNodeName()));
    $root->appendChild($doc->createElement('refund_method', $refund_method));
    $root->appendChild($doc->createElement('amount_in_cents', $amount_in_cents));

    return $this->createRefundInvoice($this->renderXML($doc));
  }