SQL Server 2019

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WITH Rewards AS (
    SELECT 1 AS CustomerID, '2010-01-01' AS DateReceived, 15 AS Point UNION ALL
    SELECT 1, '2020-01-02', 20 UNION ALL
    SELECT 2, '2020-01-08', 20 UNION ALL
    SELECT 2, '2020-01-10', 10 UNION ALL
    SELECT 3, '2020-01-10', 50
),
MaxRewards AS (
    SELECT 1 AS CustomerID, 20 AS MaxPoint UNION ALL
    SELECT 2, 10 UNION ALL
    SELECT 3, 30
),
cte AS (
    SELECT
        r.CustomerID,
        r.DateReceived,
        r.Point,
        mr.MaxPoint,
        COALESCE(SUM(r.Point) OVER (PARTITION BY r.CustomerID ORDER BY r.DateReceived
                                    ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING), 0) AS cumPoints
    FROM Rewards r
    INNER JOIN MaxRewards mr ON r.CustomerID = mr.CustomerID
)

SELECT
    CustomerID,
    DateReceived,
    CASE WHEN cumPoints + Point < MaxPoint
         THEN Point
         ELSE CASE WHEN MaxPoint - cumPoints > 0
                   THEN MaxPoint - cumPoints ELSE 0 END END AS Point
FROM cte
ORDER BY
    CustomerID,
    DateReceived;

CustomerID	DateReceived	Point
1	2010-01-01	15
1	2020-01-02	5
2	2020-01-08	10
2	2020-01-10	0
3	2020-01-10	30