Or we could put an index on the totaldue column 😉

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.Totaldue = @SPECIFIED_DIFFERENCE - TAB2.Totaldue