TestSQL1

SELECT p.client_id AS CLIENT, SUM(p.sum)-_inner.Issuance AS SALDO
  FROM processing AS p
INNER JOIN
  (
    SELECT client_id, SUM(sum) AS Issuance FROM processing
        WHERE operation_id =
                (
                  SELECT id FROM operation WHERE name = 'issuance'
                )
        GROUP BY client_id, operation_id
  )
  AS _inner
ON _inner.client_id = p.client_id
  WHERE p.operation_id =
    (
      SELECT id FROM operation WHERE name = 'payment'
    )
  GROUP BY p.client_id