Subtracting With A Condition

One of the following selected fields is a sum. I need to subtract ISNULL(R.CMS_AMOUNT,0) where R.Waived = '1
from that amount.
Pretty much just subtract those amounts where the waived field is 1. I am having trouble with the syntax. Can anyone help? Thanks.'



SELECT P.Efs_Network_ID
, F.DBA_Name
, F.ACCOUNT_STATUS
, SUM(ISNULL(R.CMS_Amount,0)) - isnull(RA.Amount,0) AS Amount -- ?? isNULL(R.CMS_AMOUNT,0) WHERE
R.Waived = '1' ??????

, isnull(SUM(RA.Pending),0) AS Pending
, F.Salesman_no
FROM tblPimsFDSLookup P
INNER JOIN ACH_MerDemo F
ON P.Efs_Network_ID = F.Efs_Network_ID
LEFT OUTER JOIN tblrecovery R
ON P.IntMid = R.IntMid
LEFT OUTER JOIN(
SELECT sum(isnull(Amount,0)) as amount
, intmid
, SUM(CASE WHEN RA.Status = 'P' THEN 1 ELSE 0 END) AS Pending
FROM tblACHRecoveryAmounts RA
--WHERE IntMid = 641046
WHERE RA.Status IN ('A','P','M','S')
Group by intmid
) AS RA
ON P.IntMid = RA.IntMid
WHERE P.INTMID = @INTMID
AND R.Waived = 0
AND Upgraded IS NULL
GROUP BY
P.Efs_Network_ID
, F.DBA_Name
, F.ACCOUNT_STATUS
, RA.Amount
,F.Salesman_no