code: public Refund refundTransaction( Transaction transaction, Integer amount ) {
      return this.refundTransaction( transaction, amount, null );
    }
docs: This function refunds a {@link Transaction} that has been created previously and was refunded in parts or wasn't refunded at
  all. The inserted amount will be refunded to the credit card / direct debit of the original {@link Transaction}. There will
  be some fees for the merchant for every refund.
  <p>
  Note:
  <ul>
  <li>You can refund parts of a transaction until the transaction amount is fully refunded. But be careful there will be a fee
  for every refund</li>
  <li>There is no need to define a currency for refunds, because they will be in the same currency as the original transaction</li>
  </ul>
  @param transaction
  The {@link Transaction}, which will be refunded.
  @param amount
  Amount (in cents) which will be charged
  @return A {@link Refund} for the given {@link Transaction}.