Michael turned up in Havana without the $2 million, the 'little gift' to Batista probably padded with Roth's commission!

Then why would Michael get Fredo to bring the money after witnessing the rebel blowing himself up and his statement “the rebels could win”

Michael already knew, he only had until the New Year's Eve to string Roth along / chance of finding out the traitor in the family [with or without the money] both Michael and Roth having made arrangements and believing the other “will never see the New Year”