UPDATE dbo.Items
>			SET Items.iqtyonhand = Items.iqtyonhand + Deleted.trn_qty - Inserted.trn_qty
>		FROM dbo.Items
>			INNER JOIN Inserted ON Items.iItemid = Inserted.trn_itmfk
>			INNER JOIN Deleted ON Items.iItemID = Deleted.trn_itmfk
>		WHERE iItemTypId = '43'

;with cteChanges AS (SELECT D.trn_itmfk AS ItemID, SUM(COALESCE(D.trn_qty,0) - COALESCE(I.trn_qty,0)) as ChangedQty
FROM Deleted D INNER JOIN Inserted I on D.trn_itmFk = I.trn_itmFK
GROUP BY D.trn_itmfk)

MERGE dbo.Items as Trg
USING cteChanges as Src ON Trg.ItemId = Src.ItemID
WHEN MATCHED and Trg.iItemTypeId = '43' 
THEN UPDATE SET iQtyOnHand = COALESCE(Trg.iQtyOnHand) + ChangedQty;