I hired a pool guy to take care of 4 pools, and send me photos of each pool being maintained each week. He did not do so, and the pools were poorly maintained and green all the time. I had to follow behind him to get photos, was unable to get paid by my client because they were not clear, and then had to redo the pools at my expense, but now the pool guy has decided to send me an invoice and demand payment! Since he did not fulfill his end of the agreement at all, I don't feel I should have to pay. I also found that he wasn't adding salt to salt pools, and wasn't going weekly like he had agreed to do, and was billing for.
You should outline what you state above in a written response and state clearly in the letter (send it in a way to ensure he gets it and you have proof of receipt) that you therefore are disputing all charges from him in good faith. If it goes to collections or an attorney, inform them that it was a good faith dispute and forward that paperwork.