SQL Server 2019

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CREATE TABLE dbo.YourTable (DateStamp date,
                            ProductID int,
                            InventoryChange int,
                            InventoryCount int);
INSERT INTO dbo.YourTable
VALUES('2021-07-01',100,-300 , 100000),
      ('2021-07-01',200,-700 , 50000),
      ('2021-07-02',100,200  ,null),
      ('2021-07-02',200,-100 , null),
      ('2021-07-03',100,500  ,null),
      ('2021-07-03',200,300  ,null),
      ('2021-07-04',100,-1000,  99500),
      ('2021-07-04',200,-100 , null),
      ('2021-07-05',100,100  ,null),
      ('2021-07-05',200,300  ,50500);
10 rows affected

WITH Groups AS(
    SELECT DateStamp,
           ProductID,
           InventoryChange,
           InventoryCount,
           COUNT(InventoryCount) OVER (PARTITION BY ProductID ORDER BY DateStamp) AS Grp
    FROM dbo.YourTable)
SELECT DateStamp,
       ProductID,
       InventoryChange,
       CASE WHEN InventoryCount IS NOT NULL THEN InventoryCount + InventoryChange
            ELSE MAX(InventoryCount) OVER (PARTITION BY ProductID,Grp) +
                 SUM(InventoryChange) OVER (PARTITION BY ProductID,Grp ORDER BY DateStamp)
       END AS InventoryCount
FROM Groups
ORDER BY DateStamp;


DateStamp
ProductID
InventoryChange
InventoryCount


2021-07-01
100
-300
99700

2021-07-01
200
-700
49300

2021-07-02
200
-100
49200

2021-07-02
100
200
99900

2021-07-03
100
500
100400

2021-07-03
200
300
49500

2021-07-04
200
-100
49400

2021-07-04
100
-1000
98500

2021-07-05
100
100
98600

2021-07-05
200
300
50800


Warning: Null value is eliminated by an aggregate or other SET operation.


DROP TABLE dbo.YourTable;

DateStamp	ProductID	InventoryChange	InventoryCount
2021-07-01	100	-300	99700
2021-07-01	200	-700	49300
2021-07-02	200	-100	49200
2021-07-02	100	200	99900
2021-07-03	100	500	100400
2021-07-03	200	300	49500
2021-07-04	200	-100	49400
2021-07-04	100	-1000	98500
2021-07-05	100	100	98600
2021-07-05	200	300	50800