Cannot Perform An Aggregate Function On An Expression Containing An Aggregate Or A Subquery.

Can any1 tell me why i am getting an error


SELECT DISTINCT

--p.voucher,

--p.amount,

p.siteID,

b.siteID,

SUM((round(b.total,2,2)) - SUM(round(p.amount,2,2))) AS OutStandingBalance,

SUM((round(b.total,2,2)) - SUM(round(p.amount,2,2))) AS CashCheque,

SUM((round(b.total,2,2)) - SUM(round(p.amount,2,2))) AS Vouchers

FROM

BillingTotal b,

Payment p

--WHERE

-- s.sitename=@cmb1

--AND p.siteid = s.siteid

-- p.voucher = 0

-- p.voucher = 1

GROUP BY p.siteID,b.siteID



Msg 130, Level 15, State 1, Line 1

Cannot perform an aggregate function on an expression containing an aggregate or a subquery.