Where was the belt fraying? The side of the belt (as if it's rubbing against something coming in/out of the B-pillar)? A specific area across the belt face or back that makes it appear fuzzy, has teeth marks like a gear may have made them, or as if someone had scraped it many little times? Is it on any area that normally touches your body when positioned in-place (like around the belt buckle area on your pants), or only an area that first pulls out and generally repositions back in the B-pillar? Or something else?
I ask because on another non-Volvo forum I used to help moderate, I remember multiple seat belt situations and over the years at least one, maybe two, where the physical safety mechanism in the B-pillar was causing the issue and needed to be replaced. It certainly wasn't a common problem with that other brand, and I of course assume your auto has not been in an accident or a situation where the belts auto-tightened (and maybe didn't even then self-release) on you. You're having the problem in the driver's seat so its obviously not caused from holding down a flawed child seat or from an attached animal harness in the back seat (yup, real examples of owner complaints in that other premium brand.) Without a picture, it may help others take a little closer notice on their Volvo belts. Good luck on the repair!
Thx for the photo. Yup, looks like what I seem to remember was happening on just one side of another guy’s Lexus years ago. IIRC, the internal mechanism in the B-pillar (and a new seat belt) ended-up being replaced for him under warranty.