Another possibility (admittedly a stretch): Michael knew the rebels would win. He knew he had to have Roth killed, but he still wanted Roth's Havana holdings. He figired the rebels would be just as open to corruption as was Batista, and would want the casinos to stay open because they brought in tourist dollars. By bringing the money, he was signaling to the about-to-be new regime that he was willing to make a big buy-in
What you say makes sense that no one would have expected the Castro rebels new regime to kill the goose that was laying the golden eggs – to shut the casinos down and following on, the tourist business bringing in the dollars
Businessmen! like Roth and Michael “willing to make a big buy-in” with any one, be it Batista or Castro for protection, so that Michael and Roth are free to make their profits
