that happened to debian. when they attempted to use some of their funds to hire developers some people got very upset.
so taking advantage of opportunities to earn money has to be carefully considered. the funds are best used for things that the community has been unable to achieve. or at least a community consensus for how to spend the money should be there, lest anyone get the impression that someone is able to enrich themselves through the project.
i can't think of a concrete example right now, but i seem to remember some projects that have literally fallen apart over issues like these.
not every project is like that of course, and debian may be the extreme here. but for most projects where this is not an issue it is because this has been clear from the start, and the kind of people for whom it would be an issue self-selected to avoid contributing to these projects in the first place.