WITH CTE
     AS (SELECT a.*, 
                ROW_NUMBER() OVER(PARTITION BY a.manager, a.id_1c, a.report_date ORDER BY a.period_id DESC) AS rn, 
                DENSE_RANK() OVER(PARTITION BY a.manager, a.id_1c ORDER BY a.report_date) AS gr, 
                b.prepaid_amount
           FROM #debt_period_client AS a
                INNER JOIN #client_debt_prepaiment AS b ON a.manager = b.manager
                                                           AND a.id_1c = b.id_1c
                                                           AND a.report_date = b.report_date
          --WHERE a.manager = 'Менеджер 1'
		  ),
     CTE2
     AS (SELECT *, 
                LAG(debt_amount_real) OVER(PARTITION BY manager, gr ORDER BY rn) AS xx
           FROM CTE)
SELECT report_date,
       manager,
	   id_1c,
	   period_id,
	   debt_amount,
	   debt_amount_real =
		 CASE 
		   WHEN xx IS NULL THEN IIF(prepaid_amount > debt_amount_real, 0, debt_amount_real- prepaid_amount  )
		   ELSE IIF(debt_amount_real > (prepaid_amount - xx), debt_amount_real - (prepaid_amount - xx) , debt_amount_real)
		 END 
FROM CTE2
ORDER BY manager, report_date, id_1c, period_id

report_date manager                   id_1c      period_id   debt_amount                             debt_amount_real
----------- ------------------------- ---------- ----------- --------------------------------------- ---------------------------------------
2021-09-01  Менеджер 1                01         2           10000.00                                10000.00
2021-09-01  Менеджер 1                01         3           30000.00                                15000.00
2021-09-01  Менеджер 1                01         4           25000.00                                0.00
2021-09-02  Менеджер 1                01         2           8000.00                                 8000.00
2021-09-02  Менеджер 1                01         3           26000.00                                4000.00
2021-09-02  Менеджер 1                01         4           18000.00                                0.00
2021-09-03  Менеджер 1                01         2           8000.00                                 24000.00
2021-09-03  Менеджер 1                01         3           26000.00                                34000.00
2021-09-03  Менеджер 1                01         4           18000.00                                8000.00
2021-09-01  Менеджер 2                04         1           2000.00                                 14000.00
2021-09-01  Менеджер 2                04         2           15000.00                                12000.00
2021-09-02  Менеджер 2                04         1           2000.00                                 11000.00
2021-09-02  Менеджер 2                04         2           10000.00                                9000.00
2021-09-03  Менеджер 2                04         1           2500.00                                 2000.00

--declare @k int = 3100
DECLARE @k INT = 31000;
--declare @k int = 310000

WITH cte
     AS (SELECT id = 1, d = 10000, k = @k UNION
         SELECT id = 2, d = 12000, k = @k UNION
         SELECT id = 3, d = 8000, k = @k UNION
         SELECT id = 4, d = 19000, k = @k),
     cte2
     AS (SELECT a.*, 
                LAG(k - dd) OVER(ORDER BY id) AS calc
           FROM cte AS a
                OUTER APPLY
         (
             SELECT SUM(d) AS dd
               FROM cte AS b
              WHERE a.id >= b.Id
         ) AS c)
     SELECT a.id, a.d, a.k,
            CASE
                WHEN ISNULL(calc, k) >= d THEN 0
                WHEN ISNULL(calc, k) < d
                     AND ISNULL(calc, k) > 0 THEN d - ISNULL(calc, k)
                WHEN ISNULL(calc, k) <= 0 THEN d
                 ELSE ISNULL(calc, d)
            END AS [То что нужно], 
            a.d - CASE
                      WHEN ISNULL(calc, k) >= d THEN 0
                      WHEN ISNULL(calc, k) < d
                           AND ISNULL(calc, k) > 0 THEN d - ISNULL(calc, k)
                      WHEN ISNULL(calc, k) <= 0 THEN d
                       ELSE ISNULL(calc, d)
                  END AS [Проверка]
       FROM cte2 AS a;

DROP TABLE IF EXISTS #T;

DECLARE @pay1 INT = 18999, -- для 1-го манагера
             @pay2 INT = 10000, -- для 2-го
	     @pay3 INT = 550   -- для 3-го

SELECT *
 INTO #T 
 FROM (SELECT mng = 1, id = 1, [долг] = 10000, [платеж] = @pay1
       UNION SELECT mng = 1, id = 2, [долг] = 12000, [платеж] = @pay1
	   UNION SELECT mng = 1, id = 3, [долг] = 8000, [платеж] = @pay1 
	   UNION SELECT mng = 1, id = 4, [долг] = 19000, [платеж] = @pay1
	   
	   UNION SELECT mng = 2, id = 1, [долг] = 1500, [платеж] = @pay2
	   UNION SELECT mng = 2, id = 2, [долг] = 500, [платеж] = @pay2
	   UNION SELECT mng = 2, id = 3, [долг] = 100, [платеж] = @pay2
	   UNION SELECT mng = 2, id = 4, [долг] = 2500, [платеж] = @pay2

   	   UNION SELECT mng = 3, id = 1, [долг] = 600, [платеж] = @pay3
	   ) a;

;WITH CTE AS
(
SELECT * 
       ,SUM([долг]) OVER(PARTITION BY mng ORDER BY Id) ss
       ,[платеж] - SUM([долг]) OVER(PARTITION BY mng ORDER BY Id) [остаток]
 FROM #T
 )
 SELECT mng, id, [долг], [платеж],
	    CASE WHEN ISNULL(LAG([остаток]) OVER(PARTITION BY mng ORDER BY Id), [платеж]) <= 0 
                      THEN [долг]
                    ELSE 
                       ABS((ISNULL(LAG([остаток]) OVER(PARTITION BY mng ORDER BY Id), [платеж]) - IIF([остаток] < 0, 0, [остаток])) - [долг])
           END [остаток]
   FROM CTE
   ORDER BY mng, id;

mng         id          долг        платеж      остаток
----------- ----------- ----------- ----------- -----------
1           1           10000       18999       0
1           2           12000       18999       3001
1           3           8000        18999       8000
1           4           19000       18999       19000

2           1           1500        10000       0
2           2           500         10000       0
2           3           100         10000       0
2           4           2500        10000       0

3           1           600         550         50