One characteristic of gangsters is that they think everybody has a price. As has been pointed out, noone knew (except Kruschev and Che) that Castro was a communist. So, Roth and Michael figured they could do business with Castro (although Roth doesn't seem to realize that the rebels can win). However, Michael wants to wait because he does think the rebels can win and the $2 million can better serve Michael's interests if it is paid to a possible Castro regime rather than to Batista in his waning days in power.