Should be straightforward from here, Bill:

DROP TABLE #Invoices

CREATE TABLE #Invoices (InvoiceNo VARCHAR(10), Amount DECIMAL(10,2))

INSERT INTO #Invoices (InvoiceNo, Amount)

SELECT '1001', $230.00 UNION ALL --

SELECT '1002', $135.56 UNION ALL --

SELECT '1003', $201.00 UNION ALL -- * Does not need to show on report

SELECT '1003CM', $201.00 UNION ALL -- * Does not need to show on report

SELECT '1004', $134.00 UNION ALL -- * Report needs to show $135 - $35 = $100 <- show total

SELECT '1004CM', $65.00 -- * Does not need to show on report

SELECT *

FROM #Invoices i

CROSS APPLY (

SELECT Credits = SUM(Amount)

FROM #Invoices c

WHERE c.InvoiceNo = i.InvoiceNo+'CM'

) c

WHERE RIGHT(InvoiceNo, 2) <> 'CM'