create table #temp
(Receipt_key varchar(10), 
Item_key varchar(10) ,
Position int);

insert #temp
values
('0001','02654',NULL),
('0001','35544',NULL),
('0002','02654',NULL),
('0002','85466',NULL),
('0002','84945',NULL)

WITH upd_recs
     AS (SELECT Receipt_key,
                Item_key,
                Row_number()
                  OVER (
                    partition BY Receipt_key
                    ORDER BY Receipt_key, Item_Key) rnk
         FROM   #temp)
UPDATE #temp
SET    Position = rnk
FROM   upd_recs a
       INNER JOIN #temp b
               ON a.Receipt_key = b.Receipt_key
                  AND a.Item_key = b.Item_key 

SELECT Receipt_key,
                Item_key,
                Row_number()
                  OVER (
                    partition BY Receipt_key
                    ORDER BY Receipt_key, Item_Key) rnk
         FROM   [My Transaction Table]