You can't buy Rome as it's the faction's capital, AFAIK.

Here's sneakiness incarnate :

First, give him Rhodes or whatever island you want him to have. The best thing would be to give him a heavily contested/dangerous/riotous province so he has to fight to keep it - the Levant post-Mongols, smack in the middle of Turkey, Constantinople, you name it. Rhodes would work too of course, but anything that will drain his treasury and military is good.

Then secure military rights from the Pope. Build lots of troops, two or three stacks worth, and send them to the Rome region without a general. Let simmer. They'll eventually turn rebel. The Pope will deplete his military to fight them, especially if they're good autocalc troops. Hopefully, by the time he's beaten them he'll be 1)at the head of a severely depleted force and 2) far away from Rome, chasing the last rebel dregs.

While he's out fighting the barbarian horde, send a gazillion spies in Rome, and use assassins to torch the law and happiness buildings (yeah, I know, but you can't make a town go rebel without the assassins). This MUST be done in one single turn, else he'll just build more militia to counter the PO threat.
Hopefully you can then be the Hero of Christendom and save Rome from this ghastly revolt... then keep it and let His Holiness rot in his dominion over Rhodes "for his protection"

Alternatively, you could also get the pope to start a war with another Italian faction, like Sicily, as has been said, and back them up financially. If the pope calls for a Crusade against them, you'll have to kill either the pope or the "rogue faction"'s king though, to make sure they're not wiped out and conquered by your land hungry neighbours.