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When the narrator is wrong about something in the book's world, it's called an unreliable narrator. When a narrator has a single point of view (sees through one character's eyes) then it's inevitable that some information is unreliable. Readers understand that.

With a 3rd person narrator you're also doing some factual description based on what the character experiences. It's fine to say that the man came forward to shake MC's hand and introduce himself as so-and-so. Because that actually happens. It's fine to describe the MC referring to the man with that name, because that also happens. Ditto for any thoughts of the MC's you share with the reader. Because of this, you can also have the narrator use the (false) name.

The only thing you want to avoid is any information that could not possibly come from the MC. And that would include knowing that the MC is being tricked.

+1 colmde. I'd say you can just be careful with your wording, so technically you did not lie to the reader. Don't have the narrator call him "the boss". I will add an example:

The fat man listened to the piece in his ear, then said, "The boss will be here in a minute. Show some respect."

MC said nothing, he just took his seat. A gray-haired man entered; MC noted not a hair out of place, manicured, his suit and shoes looked tailored and expensive.

The fat man said, "Boss," and lowered his eyes.

Finally, thought the MC. This is the man to kill.

The gray-haired man took his seat. "Alright. Here I am. You got three minutes to tell me why I shouldn't tell Bobby here to bag your head and shoot it. Will you do that for me, Bobby?"

The fat man said, "Yes sir. Three minutes."

"Good man. Wait until I tell you." He turned to meet MC's eyes. Not a blink. Completely relaxed and expressionless, a psychopath in waiting.

SHOW, DON'T TELL.

Don't name the guy, don't say he's a boss or a fake boss. Describe the encounter in sufficient detail, in such a way that the reader is convinced (wrongly) that the guy is the boss, same as the MC.

If the narrator is clearly non-omniscient, from the main character's perspective, then you describe things as the main character would describe them. The audience will not feel lied to in the reveal, so long as they know that the narration is from that perspective.

Just think of any story involving a mystery, where the narrator character thinks that X is the villain, only then its revealed that it is Y. The audience doesn't feel cheated; that's just how POV narration works.

Don't have the narrator mention the word "Boss" at all. The meeting was likely scheduled by someone, they can be the one to deceive the MC telling them, "You'll meet the boss at 123 Fake St. at 2:00", as well as maybe the fake boss themselves (by acting as if they are the boss, etc.)

Other clues that this guy is the real boss can be in the mind of the MC but not outright stated by the narrator.

In a third-person limited (or first-person) narrative, deceiving the MC and deceiving the reader are pretty much the same thing, since the reader only knows as much as the protagonist. In this case, there's nothing wrong with a little deception.

One of my favourite adventure games, Another Code: Two Memories, actually does exactly what you describe:

Midway through the game, the protagonist ''finally'' meets up with her father, which is her main objective for most of the game up to that point... except it wasn't actually him. Her ''actual'' father shows up a short time later, and the guy she met earlier turns out to be the main villain.

I didn't feel like the game had cheated me, or lied to me; I felt like it was a very clever and surprising twist.

What you could try and do is include subtle little hints that there's something not quite right about this guy. Something he says, or does, that doesn't fit the established facts. Ideally, something subtle enough that a reader won't pick up on it on first reading, but in hindsight will make them think, "Oh yeah, of course this guy was a fake!"

If you use a first-person narrator, no problem. Readers expect a first person narrator to reflect the knowledge and opinions of that person. If I was reading someone's diary and read, "I met the boss", and then later he says, "Oh, that wasn't really the boss, they fooled me", I wouldn't say he lied, just that he was mistaken.

A third person narrator can be presented as a fallible human. Like some stories will start out saying something like, "Here's the story as Jack told it to me" or some such, and then switch to third person for the bulk of the story. But the reader still identifies the narrator with the fallible human who is relating the story.

But yes, generally we expect a third person narrator to be omniscient and infallible. If in the beginning of the story you say "Jack met the boss" and later you say that that wasn't really the boss, the reader may well feel like you lied to him.

I think the solution to this is careful wording. Don't have the narrator say, "He met the boss." Have a character say this in dialog.

For example:

"It's time for you to meet the Boss," Mr Miller said.

Miller led Jack down the hall and through a door. A tall, gray-haired man was sitting behind a desk.

"Are you the Boss?" Jack asked.

"Yes, I am," he said.

Etc. I'm not claiming this is a brilliantly done scene, but note that as I wrote it, the narrator never says that this person is the boss. The narrator simply relates that Miller said this and that the man behind the desk said so.

Sometimes getting the wording right for this sort of thing is tricky. You don't want to make it obvious to the reader that you are carefully avoiding saying something. Like I'd avoid something like

Miller introduced him to the person he said was the boss.

That wording is awkward and a reader may well suspect you're avoiding saying he actually is the boss. But

Miller said, "Here's the boss."

That doesn't sound suspicious. It's exactly what you might write if the person really was the boss. You need to make it flow naturally. Later, when the reader figures out that this person was a fake, he may flip back to this scene and say, "Ohhhhh, he never actually said that this person was the boss, just that Miller said he was the boss ..." And that's fine.

So I'm sure you've read the first Harry Potter book, where Harry is absolutely convinced Snape is the villain of the piece and all evidence seems to point to this. Snape is muttering an incantation as Harry's broom goes haywire during a Quidditch Match, only to stop when Hermione sets his robes on fire and he breaks contact. Later Harry overhears Snape threaten Quirrel for not "helping him steal the stone".

Of course, it shouldn't have to be a spoiler to all reading that the real villain is Quirrel and Snape was actually never working with him (And if it is, while I'm on a role, Snape kills Dumbledore, Vader is Luke's Father, Clarence gets his wings, Rosebud was his sled, Jesus gets better, you should know all of this by now). In the first case, Quirrel was cursing the broomstick, while Snape was working the counter curse. The curse stopped when Snape caught on fire because in his surprise, he knocked Quirrel in the face, breaking his concentration on Harry's Broom... and this also allows Snape to figure out who is the villain trying to steal the stone. We also have a scene where it seems a nervous Quirrel is overheard working out his panic to himself, but even that isn't exactly what it seems.

This is a writing device known as a Red Herring, where in mystery plots, a character is designed to purposely mislead the audience into believing they are the guilty party, but when it's time for the meddling heroes and their dog to pull the mask away it turns out it's the character that the audience wasn't expecting (And in the spin off series "A Pup named Scooby-Doo" had an actual character named Red Herring, who appeared in a running gag where Fred would accuse him of being the culprit for no good reason beyond just not liking him, only for him to show up and disprove it... it was such a running gag that, Red Herring eventually was the correct culprit because the audience was so used to the joke they never suspected it would be legit).

Normally the Reds are people who have personalities that are rather standoffish to the hero but can be re-framed in a different light once we get the real bad guy. This can be as simple as not allowing the cop hero to search his residence without a warrant and being dick about it to boot. Here, the reader is lured into a false idea that "If he isn't guilty, he shouldn't have anything to hide" where as the truth is that the cop is encroaching on a secret that while dark, isn't illegal (the victim and he are in an affair and want it hidden from their mutual spouses) or he's just really into his rights and doesn't want to cops going on a fishing expedition in his domicile when they can't meet the burden of proof to convince a judge to sign off on it.).

In this case, your best bet is to tell the truth... but let the leader think you lied. So how your story plays out: The hero meets someone who is connected to the mafia boss. Hero demands this person arrange a meeting with the boss. The connection thinks it over and worries that if the boss doesn't want to, he could be killed... after some back and forth, and some heroic threats, the connection is convinced he can arrange the sit down. The connection gives the time and place and any stipulations about the meeting (come alone or unarmed) and if he follows those instructions, he can meet the boss.

The hero is there at the time and place with no boss in sight. The place is in a rather sketchy part of town and most customers, such few that there are in this place during a lunch period, glare at the hero... even the waiter is giving him looks for lingering. Finally, up drives a car and out comes the connection, who opens the door for the fake!Boss. Fake!Boss comes in and has the sit down, threatens the hero and what have you, then tells the connection to bring the car around, they're done here. And then they drive off.

At first, knowing the boss in this scene is the "boss", your first question is "How is that telling the truth? After all, the scene plays out exactly like I should, meaning the "boss" isn't the boss... you clearly lied!"

To which I take offense. You impugned my honor as a rando on the Internet. I said that if the hero came at the right time and place, he can MEET AND SIT DOWN WITH THE BOSS. I never said he would talk with the boss. See, here's the little secret of this whole bit... the boss was there the whole time. He was the man who you threatened to make the the connection. The guy he brought along, a trusted bodyguard/thug who has been working with the boss so long, he can properly ask all the questions the guy playing the boss' valet (aka the real boss) would ask if he were actually doing it. I kept my word. The hero meets with the boss. The hero sits down with the boss. I never said anything about speaking with the boss OR getting the answers straight from the boss' mouth. I gave everything I promised, it was use who didn't understand what was being agreed too.