I always thought that Roth told th Rosato Brothers it was from Micheal.
You see, the Rosatos and Pentangali pretty much hated each other. Pentangali could not kill them because it would boch up Micheal's business plans with Roth and the Rosatos could not kill Pentangali because then they would have the whole Corleone family coming after them.
But if Roth told thhe brothers that Micheal wanted Pentangali dead, then they semi-stupid Rosatos would go ahead with the hit.
At least thats how I see it.