I posted in another thread that Michael, the arch-controller, would never have let Tom arrange a murder--much less one that implicated a US Senator--on his own. I'm guessing that, since Geary was a regular at Fredo's brothel ("We done it be-FOEwer..."), his patronage was reported to Michael right away. I'm guessing that, after the "oily hair. silk suit" insult, Michael instructed Tom and Neri to nail Geary's ass the next time he showed up in Fredo's brothel. It happened to occur while Michael was gone and Tom was "the Don," so Fredo gave the order, but Michael had approved it in advance.