SELECT  c.user_id, SUM(c.amount)
FROM (
        SELECT d.user_id AS user_id, -(d.amount) AS amount
        FROM demand d
        WHERE d.cancel_date IS NULL
        UNION ALL
                SELECT p.user_id AS user_id, p.amount AS amount
                FROM payment p
                WHERE p.cancel_date IS NULL
) c
GROUP BY c.user_id