To senior researchers, how would you view a student you wanted to work with, but who could not?
Personally, I am much happier when somebody tells me "I can't do this because XXX" than when they disappear on me.  If they tell me they can't, then I can adjust my plans and act accordingly; if they don't tell me, then they leave me hanging and I find it quite frustrating.

I strongly advise you to make a clear decision about whether you are going to work on the project right now, and then go and clearly tell the person, through a direct video/phone link rather via email if possible.  If the decision is no, because your current supervisor is pushing you to focus on something else, then that will be quite understandable and will make no prejudice against you for most people---it is a quite typical difficulty for many people in graduate school.

As for whether they will be willing to recommend you as a postdoc: personally, if I like and respect a student, that's not going to be changed if external circumstances disrupt our work together.  If would not be as strong, of course, as if we had done more good work together that I could talk about more, but it would still be positive.  Others, of course, may vary in their opinions, and you can only find out from your associate by asking.