def fulfill(self, agreement_id, reward_address, amount, account):
        """
        Fulfill the lock reward condition.

        :param agreement_id: id of the agreement, hex str
        :param reward_address: ethereum account address, hex str
        :param amount: Amount of tokens, int
        :param account: Account instance
        :return:
        """
        return self._fulfill(
            agreement_id,
            reward_address,
            amount,
            transact={'from': account.address,
                      'passphrase': account.password}
        )