It'd take some crazy POD that would probably butterfly the situation, but If they did make the purchase, I doubt the Russian citizens would have the wealth or freedom to leave in great enough numbers to Slavicize it, no more than Mexico Romancized it. The U.S. would have been surely backed by France, and anyways it would eventually claim the territory in the name of manifest destiny anyways. They Russians did not have the means to attack a country on the other side of the world.
If they freed the serfs a bit earlier, perhaps the newly freed citizens would flock to the new colonies in search of the new wealth to be had. This would have slowed the growth the growth on the united States, as well as stalling the Russian Revolutions a good bit.