How to Get the SUM Value from Grouped by Last value in a table result
Based on your images, something like this should run on SQL Server 2008.  

(It's not completely tested because I didn't want to have to type all that data from your screenprints.)

;WITH _cte
AS (
    SELECT PurchaseId
        ,PurchaseDate
        ,SupplierName
        ,Amount
        ,Outstanding
    FROM BASE_Purchase
    )
SELECT PurchaseId
    ,PurchaseDate
    ,SupplierName
    ,Outstanding
FROM _cte c
WHERE PurchaseDate = (
        SELECT max(PurchaseDate)
        FROM _cte
        WHERE SupplierName = c.SupplierName
        )