SELECT
R.INDICE,
R.ID,
R.NUM,
R.MONTO,
R.SALDO,
         SUM(
    D.MONTO *
    CASE
        WHEN D.NUM=2 THEN -1
        ELSE 1
    END
   ) [SALDO]

SELECT *
FROM dbo.TuTabla A
OUTER APPLY (SELECT SUM(CASE WHEN NUM = 2 THEN -1 ELSE 1 END * MONTO) SALDO
             FROM dbo.TuTabla
             WHERE ID = A.ID
             AND INDICE <= A.INDICE) B
;

╔════════╦════╦═════╦═══════╦═══════╗
║ INDICE ║ ID ║ NUM ║ MONTO ║ SALDO ║
╠════════╬════╬═════╬═══════╬═══════╣
║      4 ║  4 ║   1 ║   100 ║   100 ║
║      5 ║  4 ║   2 ║     0 ║   100 ║
║      6 ║  4 ║   2 ║   100 ║     0 ║
║      7 ║  4 ║   1 ║   100 ║   100 ║
║      8 ║  4 ║   1 ║   200 ║   300 ║
║      9 ║  4 ║   2 ║  1000 ║  -700 ║
╚════════╩════╩═════╩═══════╩═══════╝