Lash Not The Hametz Buyer!!

Pursuant to the previous posts: Why does one who buys chometz receive malkos [as the Rambam rules in Chometz U'matzah 1/3] - it is a לאו הניתק לעשה!? The עשה of destroying the chometz [תשביתו] fixes the לאו and thus no malkos should be mandated???