In The Godfather 2 why does Frankie Pentangeli's would be assassin say “Michael Corleone says hello”?

Question

Frankie Pentangeli walks into a bar. A man puts a chord around his neck and says "Michael Corleone says hello".

Later on in the movie we find out that Roth ordered the hit as Michael confronts him about it. Why would the assassin say "Michael Corleone says hello" if Roth ordered the hit?

Answer 1

According to the Godfather Wiki, the attempt on Pentangeli's life was intended to fail.

Shortly before the Cuban fiasco, the Senate began hearings on the impact of organized crime. The subcommittee's lawyer, Questadt, was on Roth's payroll, and alerted Roth. Seeing a chance to eliminate Michael from the scene, Roth had the Rosatos try to kill Pentangeli and make him think that they did so on orders from Michael. Pentangeli tells the FBI that Michael is really a powerful Mafia leader who controls all of the gambling in North America, and has ordered dozens of murders.

Read this way, the passing policeman was intended to distrupt the murder. This would give Pentangeli a motive to betray his oath and reveal secrets about the Corleone family before the Senate committee meeting.

Answer 2

There are some theories that Hyman Roth told the would-be assassins to say that as part of a larger plan to turn Pentangeli against the Corleones, hence why Tom Hagen says that "Roth played this one beautifully", but this raises more questions about how much of what went down (the random cop walking by, and the shootout that follows) could have been planned in advance.

One thing we do know is that Danny Aiello simply ad libbed the line and Coppola kept it in the film.

Personally, I think he did it to taunt Pentangeli by making him think he'd been duped and that Hyman Roth was able to turn a failure into an almost success, but that's just me.