I don't have anything to test with, but I believe this could be better.

However, the problem is that it's still using a cartesian product to obtain the result.

SELECT TAB1.SalesOrderId AS ID1,

TAB1.TotalDue AS DUE1,

TAB2.SalesOrderId AS ID2,

TAB2.TotalDue AS DUE2

FROM SALESTABLE TAB1

JOIN SALESTABLE TAB2 ON TAB1.SalesOrderID < TAB2.SalesOrderID --Avoid half of the rows.

WHERE TAB1.TotalDue < @SPECIFIED_DIFFERENCE --This are useless unless you have negatives.

AND TAB2.TotalDue < @SPECIFIED_DIFFERENCE --Same as above.

AND TAB1.TotalDue + TAB2.Totaldue = @SPECIFIED_DIFFERENCE

I'm not sure if the conditions added will help but I'm sure the condition for the join must help the performance.