DECLARE @N_Sum_For_Split  MONEY = 4.2  -- сумма для разнесения 

;WITH t 
(	ID ,				-- идентификатор (PK)
	N_Order,			-- порядок сортировки (для нарастающего итога)
	N_SUM,				-- сумма 
	F_DOC,				-- идентификатор докуменат (FK)				
	N_Sum_Max_By_Doc	-- максимальная сумма в рамках одного документа, которая должна быть после разноски
) AS
(
	SELECT 1,0,3.5,1,4
	UNION ALL
	SELECT 2,1,1.1,1,4
	UNION ALL
	SELECT 3,2,0.8,2,0.4
),
Ost_Doc as --Ограничиваем суммы по документам согласно N_Sum_Max_By_Doc 
(
Select *,
case When N_Sum_Max_By_Doc - sum(t.N_SUM) over (Partition by F_DOC Order by N_Order, ID) < 0 Then 
          Case When N_Sum_Max_By_Doc - sum(t.N_SUM) over (Partition by F_DOC Order by N_Order, ID) + t.N_Sum > 0 
		       Then N_Sum_Max_By_Doc - sum(t.N_SUM) over (Partition by F_DOC Order by N_Order, ID) + t.N_Sum
			   else 0 End
     else N_SUM End as N_SUM_Ost
From T
),
Ost_all as --Определяем разноски на сумму @N_Sum_For_Split
(
Select *,
case When @N_Sum_For_Split - sum(t.N_SUM_Ost) over (Order by N_Order, ID) < 0 Then 
          Case When @N_Sum_For_Split - sum(t.N_SUM_Ost) over (Order by N_Order, ID) + t.N_SUM_Ost > 0 
		       Then @N_Sum_For_Split - sum(t.N_SUM_Ost) over (Order by N_Order, ID) + t.N_SUM_Ost
			   else 0 End
     else N_SUM_Ost End as N_SUM_Ost_All

From Ost_Doc t
)
Select * From Ost_all