Does the addition of soy to meat sold to restaurants—after a real sample of it has been sent—constitute fraud in weight?

Adding soy protein to meat to increase its weight before selling it to someone who is unaware of it is a prohibited deception. The fact that you send a sample to the buyer does not make this action permissible, because the experience does not mean he is aware of the addition of soy protein, as he might not realize it. However, if the buyer is aware of it and consents to it, then there is no harm in that. Deception is concealing a defect in the merchandise or the price, which, if the buyer had known about it, he would not have purchased it at that price.