Roth knew Pentangeli would be at Anthony's party, and would be contentious about Michael favoring the Rosatos over him. So, according to Roth's reasoning, Pentangeli would have a perfect motivation to try to kill Michael--and would make the perfect patsy for the hit attempt that Roth set up.

So, when Michael called on Roth in Miami and said, "Frank Pentangeli is a dead man," Roth had every reason to believe him.
But, when Michael, instead of having Pentangeli whacked, sent him to meet with the Rosatos, Roth knew Michael didn't suspect him--raising the possibility that Michael suspected Roth. So Roth instructed the Rosatos to kill Pentangeli at their meeting. He ordered Pentangeli to be killed in order to remove the Corleones' main ally and muscle in NY--and have his own allies, the Rosatos, take over. At that point, Roth was confident that once Michael landed in Cuba, Roth could use his connections in the Cuban government to squash him.

I believe by the time of their last meeting, both Michael and Roth knew each others' intentions. One of the marvels of that scene is how they held out despite that: Michael to find out who the traitor was, Roth for the $2 million.

Once again: Roth did not engineer Pentangeli's survival to screw Michael. Roth's plan was to lure Michael to Cuba, get the $2 million, and have him killed in a government car on the way home from the Presidential New Year's Eve Party. Why, therefore, would Roth try to engineer an incredibly difficult, incredibly dangerous, split-second-timed rescue of Pentangeli? So that Pentangeli could develop a grudge against Michael and testify against him at a Senate hearing weeks or months after the attempt--by which time, according to Roth's plan, Michael would already be long dead?


And suppose Roth did want to have the cops rescue Pentangeli at the last moment. How would he do it? Call his contact at the police precinct and say, "Hello, Shultz, I want you to send a beat cop to Richie's bar and have him arrive at 3:44 and 25 seconds. Not 3:44 and 15 seconds, not 3:44 and 35 seconds--3:44 and 25 seconds. Got it? Now let's synchronize watches..."

Or, suppose he was lurking outside in a phone booth: "Hello, 56th Police Precinct? Something fishy's going on in Richie's bar. Better send a cop there--but make sure he gets there at 3:44 and 25 seconds."