code: public Transaction enterOfflinePayment(final String invoiceId, final Transaction payment) {
          return doPOST(Invoices.INVOICES_RESOURCE + "/" + invoiceId + "/transactions", payment, Transaction.class);
      }
docs: Enter an offline payment for a manual invoice (beta) - Recurly Enterprise Feature
  
  @param invoiceId String Recurly Invoice ID
  @param payment   The external payment