Use correlated subqueries to compute the subtotals for each user:
SELECT id,
name,
(SELECT TOTAL(value)
FROM Debit
WHERE user_from = User.id
AND user_to = 1) -
(SELECT TOTAL(value)
FROM Debit
WHERE user_to = User.id
AND user_from = 1) AS amount
FROM User
WHERE amount <> 0;
id name amount
---------- ---------- ----------
2 Marie 19.77
3 Peter -2.45