I disagree with redonihunter. Your message says if you "purchase" the game. If you get it for free, you didn't purchase it. So you have a loophole to not allocate Steam keys to people who got it on the 100% off sale. You can export all your purchases as a .csv file and isolate those who paid money for the game.
The game has no minimum price. It is pay what you want. If this was so at the time of the sale, what was the point of the sale?
People could have gotten it for free anyways, sale or no sale. The only thing a sale for a free item does, if you give "100%" discount, is to give people purchaser status. Once you have purchased status, you keep that, even if the price goes up.