TestSQL3

SELECT _innerALL.employee_id, (_innerALL.Issuance-_innerALL.Payment) as CLIENT_DEPT
  FROM
  (
    # Сумма всех платежей и задолженностей клиентов по работникам
    SELECT p.employee_id, SUM(p.sum) as Issuance, _innerPAYMENT.Payment
      FROM processing as p
    INNER JOIN
    (
      # Сумма всех платежей клиентов по работникам
      SELECT employee_id, SUM(sum) AS Payment
        FROM processing
      WHERE operation_id =
        (
          SELECT id FROM operation WHERE name = 'payment'
        )
      GROUP BY employee_id
    )
      AS _innerPAYMENT
    ON _innerPAYMENT.employee_id = p.employee_id
    WHERE p.operation_id =
      (
        SELECT id FROM operation WHERE name = 'issuance'
      )
    GROUP BY p.employee_id
  )
    AS _innerALL
WHERE _innerALL.Issuance > _innerALL.Payment