https://wccftech.com/todays-list-of-paid-games-that-are-on-sale-at-the-play-store-including-monument-valley-and-more/

The titles are on sale for a limited time only and become full price at any moment. If you see a price next to the “install” button, without a strikethrough showing the reduced amount it isn’t on sale anymore. Once you download a paid title that’s free, it’s, yours to keep even after the promotion ends, and you can reinstall it on any other device linked with the same Google Play ID.

Related Today’s List of Paid Games That Are Free or on Sale at the Play Store Including Bridge Constructor Medieval and More