It requires that USA are at peace, that they have £70,000 in the national reserves, have at least 1 prestige and have researched Nationalism and Imperialism. It also requires that Russia owns Alaska. It is not possible to buy Alaska from any other nation than Russia.
It gives Russia the event The Alaska Purchase? where they can choose between accepting the money or keeping Alaska. If Russia is controlled by the AI, they will accept the offer 80% of the time. If Russia accepts the offer all POPs in USA will lose 1 consciousness, and Alaska will be transferred to USA as well as the cores on it. If it is declined the relation between Russia and USA will decrease by 100.
Technically the event will change the ownership of all provinces in North America, but without significant player involvement, that will virtually always be just Alaska.