Query with aggregate function

SELECT     TOP (100) PERCENT T.CustId, T.ProjectID, T.TaskID, C.Name, T.PerPost, T.TranDate, T.TranDesc, T.BatNbr, C.Status, CASE WHEN (T .drcr = 'C') 
                      THEN - (T .tranamt) ELSE T .tranamt END AS Amount, CASE WHEN (T .TranType = 'IN') THEN 'Invoice' ELSE 'Payment' END AS TransactionType, 
                      T.Acct AS AccountNum, dbo.Account.Descr AS AcctDesc,
                          (SELECT     SUM( CASE WHEN (T .drcr = 'C') THEN - (T .tranamt) ELSE T .tranamt END) AS Expr1
                            FROM          dbo.ARTran AS S
                            WHERE      (CustId = C.CustId) AND (PerPost <= T.PerPost) AND (Acct = T.Acct)) AS PTDTotal
FROM         dbo.Customer AS C INNER JOIN
                      dbo.ARTran AS T ON C.CustId = T.CustId INNER JOIN
                      dbo.Account ON T.Acct = dbo.Account.Acct
WHERE     (C.Status = 'A') AND (T.Acct = '11551')
ORDER BY T.CustId, T.PerPost