(I'm a master's student)
I had approached a professor for an opportunity to work on a project of his, which I found very interesting. When we discussed that opportunity, he deflected my questions about funding. I hadn't promised him that I'll work, but hadn't conclusively said no either, because funding was still possible.
Because of the non-commitment, I approached another professor who is ready to fund my work in their lab.
The first professor has contacted me now to ask when I'll start working. Meanwhile I know from other students that he isn't funding new students in his lab. How do I tell him, that I am no longer looking to work with him without burning bridges?
The problem is, I was the one actively seeking them out initially.
(Don't assume the prof is a reasonable person - they're a well-regarded researcher in my field though)