While considering my options, I reached out to a couple professors to inquire on their work. Ultimately, I've opted not to go to the school in question, though I would love to work with that professor in the future, if he is willing to be a co-advisor of sorts. While he has not explicitly said he is willing to work with me, I'd like to send an email informing him of my situation.
I ultimately decided to matriculate [elsewhere], but would love assist you in your future work if at all possible.
I feel this comes off as if I assume he is already willing, but am trying to express the opposite, where the burden of "willingness to collaborate" is on him? If that makes sense? Thanks in advance.