I think what you are talking about is the Pulsair system. The system injects fresh air into the exhaust manifold to help burn unburned fuel within the exhaust manifold. It has a vacuum acuated valve right at the top front of the motor. The purple thing is the vacuum line that controls the valve. If there is a bad connection there, you will have a vacuum leak and will notice poor running at low rpm.

You made me do it. I had to go out into the garage and look at my 1995 940. I have the same disk but it has a purple wire coming out of it. It is in a larger hose that is connected to the Air Filter box. My hunch is that it houses a sensor - probably a pressure or flow meter type of sensor to detect airflow in the larger tube. My hunch also is that it is a pollution control allowing air to recirculate or to vent into the cumbustion air intake and the "disc" is a sensor for flow. If you're running OK - leave it. I do have a half decent shop manual and can look it up but I cannot guarantee that this shop manual will properly ID it.

I can't figure what that part is, of course my 740 is 9 years older than yours. I would go to the repair place and quietly and gently ask what happened and explain the maintenance history, ie dealer then independant. If they take a look and make a good faith effort to repair it or make it right, then be satisfied. If not, tell them you won't be comming back and leave it at that. Don't get ugly and nasty as it won't help. I think I've been screwed over by my formerly usual indie repair place so I just won't go back. I also won't refer any more owners there. Just my humble advice, take it or leave it.