>>oops - remove aliases in the last line ORDER BY
>>
>> ORDER BY BatchId - we don't even need more orders - since we get only one record per batch
>>

>>>select * from (SELECT	r.*, 
>>>			p.PartNo, 
>>>			p.Description,
>>>			u.FullName,
>>>			w.Caption AS Warehouse,
>>>			r.Status, Row_Number() OVER (Partition by BatchID ORDER BY p.PartNo, r.SerialNo DESC) as RowNum
>>>		FROM Returns r
>>>		JOIN Parts p ON p.PartId = r.PartId
>>>		JOIN Warehouses w ON w.WarehouseId = r.WarehouseId
>>>		JOIN Users u ON u.UserId = r.UserId
>>>		WHERE r.WarehouseId = @WarehouseId AND
>>>			(@Status IS NULL or r.Status = @Status)) OrderedResultS 
>>>            where OrderedResultS.RowNum = 1
>>>		ORDER BY BatchId

>
>Still getting
>
>Msg 8156, Level 16, State 1, Procedure ap_GetReturnsForWarehouse, Line 23
>The column 'Status' was specified multiple times for 'OrderedResultS'.
>Msg 4104, Level 16, State 1, Procedure ap_GetReturnsForWarehouse, Line 23
>The multi-part identifier "OrderedResultSet.RowNum" could not be bound.

I see another problem in your query. You already got r.*, why you're getting r.Status again at the end of it?