Are you talking about one of the two threaded studs that look like they are tack welded to the fascia or the retaining 'nuts' on the actual cluster (part number 17 in the diagram?
If your issue is a broken stud that is stuck in part # 17, it shouldn't be too hard to extract the broken stud by bonding a screw onto the end of the broken stud and unscrewing it slowly - making sure that none of the bonding agent migrates to the surrounding part of #17. If you can't extract the stud, the part number for #17 is 677448 and by a miracle they still appear to be available
https://classic-volvo.com/schroef-vo...el-677448.html
If the stud broke off the fascia, I would expect that the bigger problem is re attaching a new stud. I don't see that happening without removing the dash.
I do have a complete instrument cluster that I removed from my 1971 142 E with 4.27 final drive that I would be willing to sell; but, shipping would probably be nasty and a complete working cluster might be overkill for your problem. It also may not match with your car as there were 4 different cluster versions depending on miles vs kilometers and I expect the drive ratio (or M40 vs M41).