[__cadetill]

Prueba algo así

SELECT
  A.COD_CUENTA,
  IMPORTE +
      (SELECT SUM(IMPORTE)
       FROM MOVIMIENTO
       WHERE
         CERRADO = 'N' AND
         CARGO = 'I' AND
         FECHA = Current_Date AND
         COD_CUENTA = A.COD_CUENTA AND
         IMPORTE IS NOT NULL) -
      (SELECT SUM(IMPORTE)
       FROM MOVIMIENTO
       WHERE
         CERRADO = 'N' AND
         CARGO = 'P' AND
         FECHA = Current_Date AND
         COD_CUENTA = A.COD_CUENTA AND
         IMPORTE IS NOT NULL) SALDO_CIERRE
FROM MOVIMIENTO A, ULTIMO_SALDO B
WHERE
  BORRADO <> 'S' AND
  CARGO = 'S' AND
  A.COD_MOVIMIENTO = B.COD_MOVIMIENTO AND
  A.COD_CUENTA = B.COD_CUENTA