I have an air condtion person who has taken half my money and just gives excuses why he cannot perform the work

I had a problem with my thermostat for my air and heat. I called Home choice warranty and they sent a man out. I was told I needed a new thermostat but the insurance company only would cover $400.00 of the original cost of $ 700.00 I contacted the service rep and he said he could get me a thermostat. for $700.00 but would have to charge me $50.00 to install it. The service rep has not produced a thermostat and every time I call he gives me some excuse that the delivery truck didn't show up , or something was wrong with the item. I also paid him $450.00 so he could purchase the thermostat. This is now half the cost of $900.00 for the same thermostat. Can I get him for exploting an elder as I am 79 and my husband is 83.