Postgres 9.6

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create table your_table (
 ID int, 
 Date date, 
 Target_Value int, 
 Daily_Value int, 
 primary key (ID, Date)
)
insert into your_table (ID, Date, Target_Value, Daily_Value) values
  (1, '2020-10-01', 200, 5) 
, (2, '2020-10-01', 500, 2 ) 
, (3, '2020-10-05', 600, 10) 

, (1, '2020-11-04', 200, 50) 
, (2, '2020-11-05', 500, 80) 
, (3, '2020-11-05', 600, 40)

, (1, '2020-12-06', 200, 50) 
, (4, '2020-12-06', 400, 30) 
, (5, '2020-12-07', 300, 20) 
9 rows affected

WITH CTE_TARGETS AS (
  SELECT ID
  , MAX(Target_Value) AS Target_Value
  , MIN(DATE_TRUNC('month', Date)) as month_first
  FROM your_table
  GROUP BY ID
), CTE_MONTHLY AS
(
  SELECT 
    DATE_TRUNC('month', Date) AS month_first
  , SUM(SUM(Daily_Value)) OVER (ORDER BY DATE_TRUNC('month', Date)) AS month_daily
  FROM your_table t
  GROUP BY DATE_TRUNC('month', Date)
) 
SELECT 
  TO_CHAR(mon.month_first, 'MM/YY') AS Month
, SUM(Target_Value) - month_daily AS monthly_cummulative
FROM CTE_MONTHLY mon
JOIN CTE_TARGETS tgt ON tgt.month_first <= mon.month_first
GROUP BY mon.month_first, month_daily
ORDER BY mon.month_first


month
monthly_cummulative


10/20
1283

11/20
1113

12/20
1713