1994 GMC C15 User Comment / Complaint

I just got my 2001 venture back from shop after about 11 weeks after Head and intake manifold gasket replacement. I noticed the vehicle had over 840 miles more on it then when I took it in. When I asked about it the mechanic said soory about the milage but he had trouble reseting the "MONITOR". UH................... MONITOR????????? whats a monitor and why would it take 800 miles for it to reset....Powertrain Module maybe? Isnt there a faster way??? I begged for weeks to get the van back but I guess I was....no no no....I mean the mechanic "took me for a ride" here....what would be a fair price to charge this guy for the inconvienence and wear and tear on my vehicle???? Bear In Western NY