How to Subtract in case Statment in self Table

SELECT Payment_Id, Pay_Booking_Id, Convert(varchar(50),Payment_Date,103)AS Payment_Date,Narration,Amount,

CASE WHEN Transaction_Type = 'M' THEN 'Mile Stone' ELSE CASE WHEN Transaction_Type = 'R' THEN 'Reciept' ELSE CASE WHEN Transaction_Type = 'P' THEN 'Payment' ELSE CASE WHEN Transaction_Type = 'DT' THEN 'Direct Transfer' ELSE CASE WHEN Transaction_Type = 'C' THEN 'Journel Credit' ELSE 'Journel Debit' END END END END END as Transaction_Type ,

CASE WHEN Transaction_Type = 'M' THEN

ISNULL((SELECT SUM(Adj_Amount) FROM Adjustment WHERE Adj_What_Id = Payment_Id ), 0.000)

ELSE

ISNULL((SELECT SUM(Adj_Amount) FROM Adjustment WHERE Adj_Payment_Id = Payment_Id AND Adj_What_Id = Payment_Id),0.000)

END AS Adj_Amount_OtherTxn,

0.000 AS Adj_Amount_thisTxn

FROM Payment Pay WHERE Pay_Booking_Id = @Booking_Id

 

Given Below Image Where Payment _Id  496   Which Transaction type in PAyment it is amount Subtract in Payment _Id 240 Which Transaction type  AGREEMENT  

 

Like Agreement Amt = 521807.000 

        Payment Amt = 4000.000

 

Which minus Amount shoulde be  517807.000