SELECT TAB1.SalesOrderId AS ID1, 
           TAB1.TotalDue AS DUE1,
           TAB2.SalesOrderId AS ID2,
           TAB2.TotalDue AS DUE2
FROM SALESTABLE TAB1
CROSS JOIN
SALESTABLE TAB2
WHERE TAB1.SalesOrderID <> TAB2.SalesOrderID AND
TAB1.TotalDue + TAB2.Totaldue = @SPECIFIED_DIFFERENCE

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

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 

DECLARE @Orders Table
    (OrderID VARCHAR(4), Amount MONEY)

INSERT INTO @Orders
SELECT 2122, 5400
UNION ALL SELECT '2123', 5500
UNION ALL SELECT '2124', 1500
UNION ALL SELECT '2125', 5700
UNION ALL SELECT '2126', 4500
UNION ALL SELECT '2126', 1500
UNION ALL SELECT '2127', 5200
UNION ALL SELECT '2129', 1000

DECLARE @Amount MONEY = 7000

;WITH UNIQUEnTuples (n, Tuples, Amount) AS (
    SELECT 1
        -- Add brackets around the OrderID to make later string comparison unique
        , CAST('[' + OrderID + ']' AS VARCHAR(max)) COLLATE Latin1_General_BIN
        ,Amount
    FROM @Orders
    UNION ALL
    SELECT 1 + n.n, a.OrderID + n.Tuples, n.Amount + t.Amount 
    FROM @Orders t 
    CROSS APPLY (
        SELECT '[' + t.OrderID + ']') a(OrderID)
    JOIN UNIQUEnTuples n ON a.OrderID < n.Tuples
    WHERE CHARINDEX(a.OrderID, n.Tuples) = 0 AND n < 3 AND
        n.Amount + t.Amount <= @Amount
)
SELECT *
 FROM UNIQUEnTuples
WHERE Amount = @Amount         -- The sum you're intereseted in identifying