There probably exists lots of differing experience on this, but I've found it easier to get permission to contribute paid work on a tit-for-tat basis, because it alleviates the "why should we let our competitors use our work for free" question.

I don't think that it is a coincidence that GPL led to Red Hat, a billion dollar support and services company, while BSD let to a multitude of hardware companies. The former is direct and the latter indirect profits of their respective software. The difference in worldview leads to different business ideas.