Shattered is a fork. The original Pixel Dungeon has a permissive license and allows selling modifications with source available

I've seen some apps free on F-droid and paid in Play Store. Best of both worlds!

Here's an example: https://play.google.com/store/apps/details?id=at.bitfire.davdroid

you can download it from github for free - missing the PC improvements like more quickslots and a larger inventory screen that can stay open while dungeoneering, but it's the full game: https://github.com/00-Evan/shattered-pixel-dungeon/releases

specifically those improvements are for the paid steam release.