Narrated Abu Hurairah:
That the Messenger of Allah (ﷺ) said: Do not practice An-Najsh.
[He said:] There are narrations on this topic from Ibn 'Umar and Anas.
[Abu 'Eisa said:] The Hadith of Abu Hurairah is a Hasan Sahih Hadith. This is acted upon according to the people of knowledge, they disliked An-Najsh.
[Abu 'Eisa said:] An-Najsh is when a man who knows about the goods comes to the owner of the goods to offer him more than what it is worth, doing so in the presence of the buyer. He intends to seduce the buyer while he himself does not want to buy it, rather he only wants to deceive the buyer with his offer. And this is type of deceit.
Ash-Shafi'i said: If a man commits An-Najsh the he has sinned due to what he has done, but the sale is permissible, because the buyer did not commit An-Najsh.