If I can make a suggestion - you might want to considering adding this, in addition to the JOIN between Inserted and Deleted

WHERE D.iQty <> i.IQty

(and you might even need to account for NULL values as well)

Reason for this - if the trigger fires even though a quantity wasn't changed (i.e. old value and new value are the same), the MERGE doesn't need to occur.

Also -just curious, why use a MERGE when you're only performing an UPDATE?