Caclulate Total (Mathematical operation) in SELECT statement of SQL Server

here is my Sp

I want to Display Previous Amount column 

 ((SUM(Total)+a.CreditAmount)-a.SellAmount)as previousAmount

;WITH tempSellCredit AS (


select
	tblCustomerMaster.CompanyID as CompanyID,
	 tblCustomerMaster.CustomerID as CustomerID,
	 tblCustomerMaster.CustomerName as CustomerName,
	Format(tblCreaditMaster.CreatedDate,'dd-MMM-yyyy') as Date,
	null as SellAmount,
	tblCreaditMaster.Amount as CreditAmount,
	tblCreaditMaster.Discount as discount,
	tblCreaditMaster.CreaditDesc as Descrip
	 from tblCreaditMaster
  inner join tblCustomerMaster
  on tblCreaditMaster.CustomerID=tblCustomerMaster.CustomerID
  Union
   select
   tblCustomerMaster.CompanyID as CompanyID, 
    tblCustomerMaster.CustomerID as CustomerID,
  tblCustomerMaster.CustomerName as CustomerName,
  format(tblSellMaster.CreatedDate,'dd-MMM-yyyy') as Date,
   tblSellMaster.SellAmount as SellAmount,
  null as CreditAmount,
  null as discount,
  null as Descrip
   from tblSellMaster
  inner join tblCustomerMaster
  on tblSellMaster.CustomerID=tblCustomerMaster.CustomerID
)
SELECT ROW_NUMBER() OVER (ORDER BY (CustomerID)) CustomerID
       ,'' as PreviousAmount
       ,Date
	   ,ISNULL(SellAmount,0)as SellAmount
	   ,Isnull(CreditAmount,0) as CreditAmount
	   ,Isnull(discount,0)as discount
	   ,ISNULL(CreditAmount+discount,0) as Total
	   ,Descrip
	   
FROM tempSellCredit