docs: async def get_payment_txn(self):
          """
          Retirieve the payment transaction associated with this credential. This can be used to get the txn that
          was used to pay the issuer from the prover.  This could be considered a receipt of payment from the payer to
          the issuer.
          :return:
          Example:
          txn = credential.get_payment_txn()
          """
          if not hasattr(Credential.get_payment_txn, "cb"):
              self.logger.debug("vcx_credential_get_payment_txn: Creating callback")
              Credential.get_payment_txn.cb = create_cb(CFUNCTYPE(None, c_uint32, c_uint32, c_char_p))
  
          c_credential_handle = c_uint32(self.handle)
  
          payment_txn = await do_call('vcx_credential_get_payment_txn',
                        c_credential_handle,
                        Credential.get_payment_txn.cb)
  
          return json.loads(payment_txn.decode())