Legal advice about a bartering service not provided..?

about 2-3 months ago I posted an ad online that stated - get a free car if you fix my broken car. I would provide a person with mechanical skills a free Nissan car that works if they fix my Saturn. As expected, a lot of people answered the ad but I choose one man who came over for an hour, tried to charge the battery but got no where. He was an older man who is unemployed and I was happy make a trade with him. We talked while he worked and he said he couldn't get hired anywhere (although by the looks of him, it didn't look like he tried very hard) but then at the end of the hour he popped his shoulder in the car and left. He said that his shoulder does that from time to time and he would have to take care of it. We tried to make arrangements to meet again but things didn't work and my Saturn never got fixed. Then, I lost my job, my other car broke down and we got a huge bill and we were considering selling the Saturn to make up for the debts. When I told him that he wrote me a letter stating that I should pay him for the time he couldn't work because of his injured shoulder and the time he worked on the car. I fully believe that I should pay him maybe 20-30 dollars for the hour for the labor even though he only sat around charging the battery and looked at the car; but paying him for the time he couldn't work because he admitted that no one would hire him is ridiculous to me. So my question is, am I legally bound to pay for that time after the car that he couldn't get a job? (even though he admitted it was a previous injury) and is 20-30 dollars enough to pay for the hour that he sat around my house and charged the battery? Legal advice about a bartering service not provided..?
You have no legal obligation to pay him anything at all. If he had fixed the car and you failed to give him the reward car as per ad, he'd have a claim against you. As it stands, unless there was a term in which you agreed to remunerate at a certain rate all those who simply made an effort to repair the car, you're free and clear.