Cannot Perform An Aggregate Function On An Express

as a follow on from my last post, here is my sql to give the sum of all payments we have recived for a particular batch of data , dependant upon the rate.

Select
SUM(Case dp.ReceivedByID
When 2 Then dp.PaymentAmount * (cn.DirectRate / 100)
When 1 Then dp.PaymentAmount * (cn.FeeRate / 100)
Else dp.PaymentAmount * ((cn.FeeRate + cn.FieldRate) / 100)

End)
From
dbo.DebtPayment dp,
dbo.ImportBatchItem bi,
dbo.Debt d,
dbo.Contract cn
where
cn.ClientContractID=d.ContractID
AND d.Debtid=dp.DebtID
AND dp.DebtID=bi.ItemID
AND bi.ImportBatchID=101
AND cn.ClientID=1021



cn.DirectRate is the current contract rate. This rate can be changed at any time on the system. i have created a table that stores the the rate for a particulat time frame. with the sql to return ruturn the rate for a time period, client & contract like so.

select cfc.ComRate from dbo.mContractFeeChange cfc
where cfc.ClientID=1021
and cfc.contractid=12
and someDate >= cfc.StartDate AND someDate <= cfc.EndDate

when i plum this as a sub-query into my main query as so



Select
SUM(Case dp.ReceivedByID
When 2 Then dp.PaymentAmount * (

(select cfc.ComRate from dbo.mContractFeeChange cfc
where cfc.ClientID=1021 and cfc.contractid=12
and dp.PaymentDate >= cfc.StartDate AND dp.PaymentDate <=cfc.EndDate)/ 100)

When 1 Then dp.PaymentAmount * (cn.FeeRate / 100)
Else dp.PaymentAmount * ((cn.FeeRate + cn.FieldRate) / 100)

End)
From
dbo.DebtPayment dp,
dbo.ImportBatchItem bi,
dbo.Debt d,
dbo.Contract cn
where
cn.ClientContractID=d.ContractID
AND d.Debtid=dp.DebtID
AND dp.DebtID=bi.ItemID
AND bi.ImportBatchID=101
AND cn.ClientID=1021



I get the following error.

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


any help most welcome