Problems after a thermostat replacement

I had the problems with my van producing heated air during the past winter months. The cool air worked fine so I took it to a local repair shop for service. They examined my van and detailed a work order for replacement of my car's thermostat in the engine. The work was done and I continued to use the van, now with working heat. I did have one episode where my car overheated while using the maximum heater temp but otherwise no problems. Now that it is summer, I have tried to use my cool air (previously working prior to thermostat replacement) and now it doesn't cool. I called the garage for help and the mechanic who had done the work said my cool air was not working now because of having to replace the thermostat. He offered to fix the problem for an additional $200. I am skeptical about this since the cool air worked fine prior to his meddling with my engine and now I have pay to fix a problem that he caused? Is this normal or is the mechanic trying to take advantage of me?