If you can tell us what code(s) you have, we can go from there as it's impossible to guess otherwise.
As for a bad MAF sensor, highly unlikely on an 850. A code for that is usually a connection problem and simply cured by using a plastic tie strip to keep its connector from moving.

If your mechanic charged you money, why not take it back? I'd suspect that he has little experience working on newer Volvos like 850s, probably knows RWDs better so another educated, experienced tech might be a better option for future referance.