CASE
	      WHEN p.N_Amount > Долг AND ROW_NUMBER() OVER (PARTITION BY F_Subscr, YEAR(D_Date), MONTH(D_Date) ORDER BY LINK) = 1
		  THEN p.N_Amount - Долг
		  ELSE CAST(CAST(p.N_Amount AS FLOAT)*b.N_Amount/NULLIF(Долг,0) AS DECIMAL(15,2)) END Распределение 

DECLARE @LINK INT 
SET @LINK =1;
With Payments 
AS ( SELECT *
     FROM dbo.FD_Payments
	 WHERE LINK = @LINK),
     Bills 
AS ( SELECT *,
            ISNULL(SUM(N_Amount) OVER (PARTITION BY F_Subscr, YEAR(D_Date), MONTH(D_Date)),0) AS Долг
	 FROM dbo.FD_Bills
     WHERE F_subscr IN (SELECT F_Subscr FROM Payments) AND N_Amount <> 0)
SELECT p.LINK as F_Payments, b.LINK as F_Bills, C_Sale_Items, p.N_Amount, b.N_Amount,
       CASE
	      WHEN p.N_Amount > Долг 
		  THEN p.N_Amount - Долг
		  ELSE CAST(CAST(p.N_Amount AS FLOAT)*b.N_Amount/NULLIF(Долг,0) AS DECIMAL(15,2)) END Распределение  
FROM Payments p JOIN
	 Bills b ON p.F_Subscr = b.F_Subscr