WITH Orders AS (
SELECT 1 id, CAST('01.10.2020' AS date) dt, 1 AS id_cust, 150 AS summ
UNION ALL
SELECT 2 id, CAST('03.10.2020' AS date) dt, 1 AS id_cust, 400 AS summ
UNION ALL
SELECT 3 id, CAST('04.10.2020' AS date) dt, 1 AS id_cust, 500 AS summ
UNION ALL
SELECT 4 id, CAST('02.10.2020' AS date) dt, 2 AS id_cust, 1000 AS summ
), Pays AS (
SELECT 1 id, CAST('02.10.2020' AS date) dt, 1 AS id_cust, 500 AS summ
UNION ALL
SELECT 2 id, CAST('03.10.2020' AS date) dt, 2 AS id_cust, 900 AS summ
), Pays_All AS (
SELECT id_cust, sum(summ) AS summ FROM Pays GROUP BY id_cust
), Orders_1 AS (
SELECT id, dt, id_cust, summ, row_number() OVER(PARTITION BY id_cust ORDER BY dt) AS rn FROM Orders
), R0 AS (
SELECT O.id, O.dt, O.id_cust, O.summ, P.summ - O.summ AS Ost, rn
FROM Orders_1 AS O  
 	LEFT JOIN Pays_All AS P
  		ON P.id_cust = O.id_cust
WHERE rn = 1
UNION ALL
SELECT O.id, O.dt, O.id_cust, O.summ, R0.Ost - O.summ AS Ost, O.rn
FROM Orders_1 AS O
  INNER JOIN R0
  	ON R0.id_cust = O.id_cust
  		and R0.rn + 1 = O.rn
) SELECT id, dt, id_cust, summ,
	case
    	when Ost >= 0 then 0
        when ABS(Ost) <= summ then ABS(Ost)
        else summ
    end AS dolg
FROM R0 
ORDER BY id