Can I make a paid app on playstore free on fdroid ?

But be careful in case others worked or contributed to the project as well. If you operate against their will, then it can get into the unethical part, but that depends on the context and I'm not a lawyer anyway. But if you are the only developer, then you pretty much do whatever you want, if the license of the libraries you are distributing allow it. I mean there are sometimes libraries that do not allow making money off, which would be a violation of the terms of its license, if you sell it with your program.