Don't you think America has been already been discussed between Khaldoon, Soriano, Txiki, Messi's father, and Messi?
City will have already put forward a long-term financial proposal and package to Messi and his father, which includes playing for City, and then New York City.
Messi would be the figure head of the City Football Group, Manchester City, and New York City.
Obviously I have no idea, but I would assume Sheikh Mansour is also very much involved in City and the City Football Group signing Messi. After all, he is the majority share holder, and has a personal wealth of £20bill.
The only thing stopping City signing Messi is either Messi himself or his Wife, who may not want to move to Manchester, due to the cliamte and lifestyle.