Is selling medium-quality bricks at the price of premium bricks, with the seller's knowledge and without informing the buyer, actually prohibited, and what must be done to rectify this matter?

What the questioner did by selling medium bricks as excellent without clarification is considered cheating, because cheating is creating the illusion that a desired characteristic exists in the sold item, or concealing the absence of an existing characteristic. He must repent and return the difference in price between the two types to the buyer. If he cannot identify the buyer, he should donate the amount on the buyer's behalf.