I’d say one reason Mikey didn’t hand the money over right away was to string along Roth because he knew if he paid him right away when they met then their plan is in motion to kill him. The longer he strings Roth along the more time he buys himself no matter what. Just him slow playing his hand while trying to figure out who turned on him.

Also he had to bring the money I think because Roth had people watching as he mentioned Mike bringing the money to the island. If it doesn’t turn up then he’ll think for sure there is no deal