I majored in International Affairs and have my law degree. He's got no choice but to return these properties. It's one thing to revoke diplomatic immunity; it's another entire matter to seize diplomatic property. Russian consulates, embassies, and general diplomatic missions, regardless of the fact they're in the United States are Russian Property and fall under all Russian laws.
Having went to college in DC, it's why us underage college kids used to love the Russian Embassy on Halloween. All the kids from GW & Georgetown would line up outside the gate to walk through to get our free shots of vodka, which was perfectly legal because on Russian territory, the drinking age wasn't 21.
Essentially, seizing a diplomatic outpost is considered a hostile act, even an act of war under diplomatic protocol. Putin opted not to respond to Obama's taunt because he figured Obama was going to be out soon enough, but if they're not returned, he's going to pick the best American real estate in Russia and return the favor. This is a suck it up buttercup moment, nothing conspiratorial.