His speech was poorly prepared, and I soon knocked down his argument.
понижать, сбивать цены.
en (экон.)
The price was knocked down to $3.
продавать, загнать с аукциона; ударом молотка объявить товар проданным.
In that case, the printed catalogue at an auction sale stated that each lot was subject to a reserve price, but the auctionneer knocked the goods down to the plaintiff for less than the reserved price by mistake.