https://wccftech.com/how-to/todays-list-of-paid-apps-that-are-free-at-the-play-store-including-neo-monsters-and-more/

The titles are free for a limited time only and become paid at any moment. If you see a price next to the “install” button, it isn’t free anymore. Once you download a paid title that’s free or on sale, 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 Apps That Are Free or on Sale at the Play Store Including Crypto Helper and More