Return NON-Matched values from the result of XML path - sql

Question

I have a situation where I need to compare two tables where t1.ColumnA = t2.ColumnA and t1.ColumnB<>t2.ColumnB. The caveat to this problem is that t2.ColumnB is using a "FOR XML PATH" to concatenate like values from another table (linked server using OPENQUERY). This all has to be done within a View.
To concatenate the rows, I am using the following code:
SELECT DISTINCT
CAST(A.CHECK_NUMBER AS nvarchar) [CHECK_NUMBER]
,(
SELECT B.INVOICE_NUMBER + '|'
FROM OPENQUERY([SERVER], 'SELECT * FROM CHECK_LISTING ') B
WHERE B.CHECK_NUMBER = A.CHECK_NUMBER
AND (NULLIF (B.INVOICE_NUMBER, '') IS NOT NULL)
FOR XML PATH('')
) [INVOICE_NUMBER]
, (
SELECT LTRIM(RTRIM(B.PURCHASE_ORDER_ID)) + '|'
FROM OPENQUERY([SERVER], 'SELECT * FROM CHECK_LISTING ') B
WHERE B.CHECK_NUMBER = A.CHECK_NUMBER
AND (NULLIF (B.PURCHASE_ORDER_ID, '') IS NOT NULL)
FOR XML PATH('')
) [PURCHASE_ORDER_ID]
FROM OPENQUERY([SERVER], 'SELECT * FROM CHECK_LISTING ') AS A
This works perfectly and concatenates just like it needs to. My problem is that my next view I created was to run that view against a local table to see the difference in the INVOICE_NUMBER.
SELECT
A.EntryID,
A.Check#,
A.CheckDate,
A.CheckAmount,
A.VendorID,
A.VendorName,
B.INVOICE_NUMBER,
A.Invoice#
FROM dbo.APChecks AS A
LEFT JOIN
dbo.CHECKS_Step2 AS B
ON A.Check#=B.CHECK_NUMBER
WHERE (A.Invoice# != B.INVOICE_NUMBER)
When I try to run this, the query takes at least 25+ minutes. I had to stop the query manually. Some concatenated values are longer than 1000 chars. I was told that it was not possible to INDEX on a dynamic entry like an XML PATH.
Any suggestions? Thank you in advance.