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()

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())