Intentional deception is a smaller wrong than misrepresentation or fraud, but it can erode the negotiation by undermining trust and good faith: cornerstones of any transaction. They happen most often when the speaker lies to themselves, just a little. Without a level of trust, there is no commerce.
Examples are the “little white lies” that negotiators tell. They may put it down as a “bluff” or thinking it is part of the game (“all negotiators lie”). Saying, “I do not truly know anything about that” is an example. Notice the cognitive figleaf place in can be a tell - what does “truly” mean? The speaker may think, “truly” means absolute knowledge and few can claim absolute knowledge. Professionals in the persuasion business, such as lawyers, diplomats, and politicians, can ruin careers or be sanctioned if they lie or distort. Lawyers are ethically mandated to tell the truth. They are not obligated to tell everything they know, however, as to do so would violate client trust and undermine advocacy. Lying undermines constructive negotiation, and is never appropriate. Say nothing instead of lie.
White lies can quickly turn into darker ones, so the behavior must be corrected. Call it out, politely at first. Learn how to detect deception as best you can, knowing that most people won’t spot small lies. “Question five times,” as the Japanese saying goes.