Am I obligated to go to mediation?

I sold my house last year and never had water in the basement. On the seller's disclosure I stated that I had a little moisture 1 time which is the truth. The buyer wants to go to mediation and is asking me to pay for repairs he did after he bought the house. He wants me to pay half the mediator's fees upfront. I did agree to go to mediation on the seller's disclosure. I do not see why I should pay half of these fees if I did nothing wrong and he had water damage after he bought the house.