There was a book by Jeffrey Archer called A Matter of Honor that centered on a cat and mouse game between a Soviet agent and a British veteran across France and Switzerland over ownership of a painting containing the deed to the Alaska Purchase, with the idea being that if the Soviet Union could get its hands on it they could legally buy back the state, which America had just installed most of its missile defense system in, but that was just a novel and the more you think about that idea the more ridiculous it becomes.
As far as I know, the Alaska Purchase was a straight up sale, needed by the Russian government to generate funds and also to make sure that Alaska didn't fall into British hands at a time when Russia and the UK were engaged in a Cold War of sorts.
I don't think it will ever be an issue. Alaska was never an integral part of the Russian Empire and it's been a US state with a relatively large American population since the 1950s. Russia has its hands full just with the Far Eastern territories--even if there was some legal grounds for the territory reverting to their hands they have no stomach for expansion in that direction.