T-SQL Running Total Query....

I'm trying to create a sql script/view that looks at a customers Cash Receipts and then identifies how the Payment should be applied to the customers oldest (by date) invoices.  I'm having 2 issues with the below query: 1) if the customer has more than 1 Cash Receipt they are not sequentially being applied/reducing the invoices in order and 2) the apply amount doesn't seem to be correct when the customer has either 2 payments or 2 open invoices.

This is what I'm currently getting as a result set:



Basically what I would want to happen is as follows:

WITH CTE AS (
SELECT
rownum = ROW_NUMBER() OVER (Partition by CR.CUSTNMBR order by INV.DOCDATE)
, CR.CUSTNMBR as Customer_Nbr, CR.DOCDATE as Apply_Date
, CR.DOCNUMBR as Source_Document_Nbr, CR.CHEKNMBR as Check_Nbr
, CR.CURTRXAM as Available_Apply_Amount
, INV.DOCNUMBR as Apply_to_Document_Nbr, INV.CURTRXAM
, INV.DOCDATE
, case when CR.CURTRXAM - sum(INV.CURTRXAM) OVER (Partition by CR.CUSTNMBR order by INV.DOCDATE, INV.DOCNUMBR) < 0 then 0 else CR.CURTRXAM - sum(INV.CURTRXAM) OVER (Partition by CR.CUSTNMBR order by INV.DOCDATE, INV.DOCNUMBR) end as AvailableBalance
, CR.BACHNUMB AS Batch_Id
From RM20101 CR (nolock)
       JOIN RM20101 INV (nolock) on CR.CUSTNMBR = INV.CUSTNMBR --AND CR.CURNCYID = INV.CURNCYID
Where LEFT(INV.TRXSORCE,4) = 'SLST'
AND INV.RMDTYPAL = 1 
AND INV.CURTRXAM <> 0
AND CR.CURTRXAM <> 0
AND CR.RMDTYPAL = 9

) 
SELECT 
CTE.Customer_Nbr, CTE.Apply_Date, CTE.Source_Document_Nbr, CTE.Check_Nbr, 
CASE when prev.AvailableBalance is null then CTE.Available_Apply_Amount else prev.AvailableBalance end as Available_Apply_Amount,
              CTE.Apply_to_Document_Nbr, 
              CTE.CURTRXAM, 
              CTE.DOCDATE
, CASE when prev.AvailableBalance is null then 
CASE when CTE.Available_Apply_Amount < CTE.CURTRXAM then CTE.Available_Apply_Amount 
ELSE CTE.CURTRXAM 
END else prev.AvailableBalance 
END as Apply_Amount
FROM CTE
LEFT JOIN CTE prev ON prev.rownum = CTE.rownum - 1 AND CTE.Customer_Nbr = prev.Customer_Nbr
WHERE CASE WHEN prev.AvailableBalance is null then 
CASE WHEN CTE.Available_Apply_Amount < CTE.CURTRXAM then CTE.Available_Apply_Amount 
ELSE CTE.CURTRXAM 
END ELSE prev.AvailableBalance END <> 0

Suggestions on order/etc.?