DECLARE @N_Sum FLOAT = 7.3  -- сумма для разнесения

;WITH t (ID,N_SUM,d_post_date,n_period) AS 
	(
		SELECT  1 ID,		-- PK
				2 N_SUM,	-- сумма
				CONVERT(SMALLDATETIME,'20171001') d_post_date, --  дата
				201710 n_period   -- период (для упрощения)
		UNION ALL
		SELECT 2 , 1 N_SUM, CONVERT(SMALLDATETIME,'20171002') d_post_date,201710 n_period
		UNION ALL
		SELECT 3 ,  3 N_SUM, CONVERT(SMALLDATETIME,'20171102') d_post_date,201711 n_period
		UNION ALL
		SELECT 5 ,  -4 N_SUM, CONVERT(SMALLDATETIME,'20171103') d_post_date,201711 n_period
		UNION ALL
		SELECT 4 , 5 N_SUM, CONVERT(SMALLDATETIME,'20171203') d_post_date,201712 n_period
		UNION ALL
		SELECT 6 LINK, 1 N_SUM, CONVERT(SMALLDATETIME,'20171204') d_post_date,201712 n_period
	
	)
	SELECT	t.*,
			CASE	WHEN t.sum_by_step < t.n_need AND t.sum_by_step <= sum_by_period  THEN t.N_SUM 
					WHEN t.sum_by_step < t.n_need AND t.sum_by_step > sum_by_period  THEN t.sum_period 
					WHEN t.sum_by_step >= t.n_need AND t.N_SUM >= t.n_need AND t.n_need <= t.sum_period THEN t.n_need
					WHEN t.sum_by_step >= t.n_need AND t.N_SUM >= t.n_need AND t.n_need > t.sum_period THEN t.sum_period
					WHEN t.sum_by_step >= t.n_need AND t.N_SUM < t.n_need  and t.N_SUM <= t.sum_period THEN  t.N_SUM
					WHEN t.sum_by_step >= t.n_need AND t.N_SUM < t.n_need  and t.N_SUM > t.sum_period THEN  t.sum_period
			END   n_quant
	FROM (
			SELECT	T.*,
					@N_Sum - ISNULL(LAG(t.sum_by_step) OVER (ORDER BY d_post_date,N_SUM DESC ,ID),0) n_need  -- необходимое кол-во
			FROM  (
				SELECT	*,
						SUM(N_SUM) OVER (ORDER BY d_post_date,N_SUM DESC ,ID)			sum_by_step, -- нарастающий итог
						SUM(N_SUM) OVER (ORDER BY n_period )							sum_by_period, -- нарастающий итог в пределах периода
						SUM(N_SUM) OVER (PARTITION BY n_period ORDER BY n_period)		sum_period -- сумма по периоду
				
				FROM T
				WHERE N_SUM <> 0
				) t	
			WHERE t.sum_period > 0	
		) t

id n_quant
1   2
2   1
4   4    -- не 5, а 4  потому что за прошлый период сумма = -1
6   0.3