If a person accepts payment, can they change terms?

my puppy ate some electrical cords around my ac unit. I found an ad on craigslist to trade labor. I offered a lap top in return for electrical work. The man admitted he did not know anything about air conditioners but would have a look. He came out and looked, advised me it was not my ac wires. Went in, turned on my ac and it worked fine. I gave him 40 dollars for looking. He accepted and left. His wife two hours later sent me an email saying I owed them a laptop or $185 for the service call. On their posting, they state that a free estimate will be given if nothing is to trade. Thoughts?