public Refund refundTransaction( String transactionId, Integer amount ) {
    return this.refundTransaction( new Transaction( transactionId ), amount, null );
  }
public Refund refundTransaction ( String transactionId , Integer amount ) { return this . refundTransaction ( new Transaction ( transactionId ) , amount , null ) ; }
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 transactionId
Id of {@link Transaction}, which will be refunded.
@param amount
Amount (in cents) which will be charged.
@return A {@link Refund} for the given {@link Transaction}.