Roth's plan was to have Michael killed by Batissta's military after the New Year's party, and at that point the money would either be in his hands or in Fredo's after Michael's death. Fredo would never openly defy Michael by giving the money to Roth while Michael was still alive. At the heart of Fredo's treachery, at that point in the story, was his keeping it from Michael, not putting it on display.
Yes!
Roth had no organization of his own, other than Johnny Ola. Even though he had Fredo in his pocket, he had to hide his involvement in Michael's death. He had to protect himself from possible retaliation by Neri, Rocco, Tom or others loyal to Michael--or to others who'd want to grab Michael's empire and eliminate him from contention.
The birthday party scene was an attempt to provide him with an alibi: "What? Me kill Michael? Why, I loved the kid like he was my own son. I was preparing to hand over my whole Havana gaming empire to him--just ask any of those thugs who were at my birthday party!" The assassination plan was a clever ploy: Michael'd be driven to his hotel in a military car. On the way, the car would be attacked by agents of SIM (Batista's secret police) with military weapons, making it look like an ambush by Castro's guerillas. "Poor Mike, he got caught in the crossfire..."
So, it was
essential for Michael to have turned over the $2 million
personally to Roth--it would be one more very important proof that their "deal" was still valid. Having Fredo turn over the money, and then killing Michael, would have tipped Roth's hand. It also would have tipped Fredo's hand in the plot--and he needed protection against retaliation, too.
