With Google’s recent requirement for developers to shift to Play Billing Library v3, developers with open-source projects on F-Droid are facing a problem. Per XDA Recognized Developer M66B, the developer of NetGuard and FairEmail, developers who publish apps on both Google Play and F-Droid will need to start building a separate version of their app without the Play Billing Library.
https://www.xda-developers.com/f-droid-android-apps-google-play-purchases/