WITH t AS (
SELECT 'aaa' item, '2024-07-01' due_dt, 500 pnt UNION ALL
SELECT 'aaa' item, '2024-08-01' due_dt, 400 pnt UNION ALL
SELECT 'aaa' item, '2024-09-01' due_dt, 100 pnt )
SELECT item, due_dt, pnt, 
		case when SUM(pnt) over(order by due_dt asc) - 600 < 0 then 0
			 else SUM(pnt) over(order by due_dt asc) - 600
		end remain
FROM t
ORDER BY due_dt asc;

WITH t AS
(
SELECT 'aaa' item, '2024-07-01' due_dt, 500 pnt
UNION ALL SELECT 'aaa', '2024-08-01', 400 pnt
UNION ALL SELECT 'aaa', '2024-09-01', 100 pnt
)
SELECT item, due_dt, pnt
     , CASE WHEN 600 > s THEN 0
            WHEN 600 > s - pnt THEN s - 600
            ELSE pnt END remain
  FROM (SELECT item, due_dt, pnt
             , SUM(pnt) OVER(PARTITION BY item ORDER BY due_dt) s
             , 600 v
          FROM t
        ) a
;