The problem is a tiny little gear in the odometer assembly has broken some teeth. The speedometer will still indicate how fast you are going but, your miles won't count, right?? This is a very common 850 problem.
I just replaced mine myself a few weeks ago. Go to www.ipdusa.com (they are a very popular Volvo parts supply place), and click on "Volvo Products" and then click on "Interior Accessories" and scroll down to "Odometer repair kit". For $50 they sell you the little gear and a good DVD showing you how to do it. It takes about an hour with intermediate skill level of working on things mechanical. May the luck of the Irish be with you!:-)

P.S. A speedo repair shop will charge $150-$200 and you will have to ship the cluster to them.

You need to pull the cluster out, disassemble it, remove the stepper motor for the odometer, replace the little gear that is stripped out, put it all back together. Look here:

http://www.volvospeed.com/vs_forum/index.php?showtopic=8398

Whether or not you can repair it is up to you, but it really is not that tough. Although you can charge what you want the guy marketing the gear is a bit of a gouge. I get the same gear for half that price and if I take my odometer heads into my local speedo shop the guy will install, inspect, check calibration, and stand behind his work for a whopping $25.00!!! If I did not have to drive over there everytime to get one done I would have him do every single one that comes into my shop.