Is there a difference between an ענין and a מנהג? If so, what is it?
Examples differentiating the concepts would be useful.
The way I've always heard them used is akin to the difference between a chiyuv, requirement, and a chumra, a stringency.
So a minhag would be more binding, and an inyan is more voluntary.
There is a minhag to do xyz, so make sure to do it, vs there's an inyan to do abc, so it's nice to do.
(As a side point most times that people talk about something being an inyan, it is something they have no comprehension about the action or the reasoning behind it. Their use of inyan makes a minhag logical.)