The US bought the territory of Alaska from Russia in 1867.
I once read a (fictional) story based around the idea that, at the time, the agreement with Russia included a clause allowing Russia to purchase the territory back (after 99 years) at a then considered astronomical figure ( something like 1000 times the original $7.2 million paid). No one ever believed there would be that amount of money available for them to do so. But this then give Russia the option to re-own Alaska in 1966 for a now affordable £7.2 billion.
The question is – was the idea behind this entire plot in the imagination of the author, (whom I shutter to think may have been Jeffery Archer) or was the story based around an actual possibly that had once existed in the original documents and purchase agreement? In short, did Russia ever have a legal possibility of reclaiming Alaska from the US?