Roth's top priority was to get rid of Michael to stop him from encroaching on his gaming empire. His Plan A was to have Michael whacked at Anthony's party, and make it look as if Pentangeli was behind the killing. The $2 million wasn't part of Plan A, although Roth (or Ola) probably would have told Michael that he'd have to present $2 million "as a gift o the Cuban President" to seal the deal--it would add credibility to Roth's "promise" to hand over his Havana holdings.

But, when Plan A failed, Roth went to Plan B: bring Michael to Havana and have him whacked there. Now the $2 million became all-important because Roth would have to pay all or part of it to Batista to have him arrange for Michael to be assassinated in a military car taking him back to his hotel after the New Year's Eve party.