It depends on how the sale is set up. Sometimes you can only download it, not claim it.
Edit: more exactly if you're allowed to actually claim the game, then you'll own it and won't have to pay once the sale is over. If not, you'll be able to download it for the moment, but you won't own it, so hang on to your download, because after the sale is over you'd have to pay for continued access. Hope this helps.