Allah's Messenger (ﷺ) forbade the practice of An-Najsh.
A man mentioned to the Prophet (ﷺ) that he had always been cheated in bargains. The Prophet (ﷺ) said, "Whenever you do bargain, say, 'No cheating.'"
The Prophet (ﷺ) said, "For every betrayer there will be a flag by which he will be recognized on the Day of Resurrection. "