You're incorrect on that. Volvo has known about that ABS module problem (internal problem) for years now and when the unit is replaced, the problem is fixed. I've been working for Volvo dealers for too many years to think otherwise. I've never seen one of our many customers ever return with an ABS problem after replacing the control unit, '98 X70s especially and it being under a one year parts warranty, they'ld be back IF the control unit didn't fix it.
The codes are always rear wheel sensors, usually 2-3 for the RR sensor or the pump unit (which always works during a pump activation test) and if either pump or rear wheel sensors are replaced, the car will return with the same fault codes until the control unit is replaced. I've read that here too btw, misdiagnosed (sensors and/or pump motor replaced).
As I've mentioned here before, IF it's a '96 850 with front ABS sensor codes, that's yet another issue, a wiring problem addressed by an old TSB.