I don't think I've ever seen a free font with a pukka licence where that licence allows the sale of the font. It's free to you; you can't sell it on. You would be profiting from someone else's work which they have provided for free.
Many "free font" sites compound this issue by providing rip-offs of commercial fonts. So you would be profiting from others' work and the rightful recipient of royalties wouldn't be getting anything.
What you can do is charge for the ability to use other fonts in your app and then provide (separately) free font packs for it. Again, those fonts would have to have a licence which allowed you to distribute them separately from the original provider, even if you were doing it for nothing.
[StackExchange is not a legal service. This is my opinion and not legal advice. If you want legal advice, pay a lawyer.]