CREATE FUNCTION CombineValues
(
    @Item_ID varchar(5)-- The foreign key from TableA which is used 
               -- to fetch corresponding records
)
RETURNS VARCHAR(8000)
AS
BEGIN
DECLARE @SomeColumnList VARCHAR(8000);

SELECT @SomeColumnList =
    COALESCE(@SomeColumnList + ', ', '') + CAST(InvID AS varchar(20)) 
FROM tblinvoice C
WHERE C.ItemID = @Item_ID;

RETURN 
(
    SELECT @SomeColumnList
)
END


SELECT ItemID, dbo.CombineValues(ItemID), SUM(amount) FROM tblinvoice
group by ItemID, dbo.CombineValues(ItemID)