If you own the copyright then yes this is 100% legal.

There are already apps that are like this. They usually add a couple features to the paid release so that people feel like they are getting something extra for the money. The good ones will eventually move those features to the open release eventually. However, this incentivizes keeping part of the app closed source so that nobody can just rename and re-release the paid version.

It is 100% up to you for how to handle these tradeoffs. Personally, I think so long as you are principled and ready for some criticism - and can handle it gracefully - getting paid for work that builds your open source app is a very good idea. We don’t all have the luxury of maintaining high quality unpaid side projects!

Is this your app? If yes then sure, it’s your app you can do whatever you want with it. If you are still morally troubled then just offer a free apk for those who don’t want to pay but also don’t want fdroid (for whatever reason there is, I don’t know of any).

I can think of a few apps that do this. It is certainly possible. I think it is ethical; if someone is not participating in the open source community, they miss out on the benefits. I think most people involved do contribute in some way. If someone just wants to use Google for the benefit of ease and discoverability, then they can pay for it. You’re still offering an ad-free app (presumably) and adding use value. It’s perfectly reasonable to suppliment the cost of development in is way.

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