What is the ruling on changing the color of some fish, with the customer's knowledge, in order to obtain high prices?

It is permissible to sell fish that has changed color to a buyer if the buyer is aware of it and does not intend to deceive others with it. However, if the seller knows or strongly suspects that the buyer intends to deceive others with it, then it is forbidden for him to sell it to the buyer, because it involves cooperation in sin.