You may want to get into a conversation with the broker as well as the agent. I would very specifically state that you were under the understanding that you were represented by this agent. If so you believed you had Obedience, Loyalty, Disclosure, Confidentiality, Accountability and Reasonable care due to you. Then Based on the texts/ emails sent you do not have the feeling they were working in your best interest. You would like to schedule a meeting with the broker to discuss this incident. When you walk in, ask them who they are representing during this meeting you or the buyer? Proceed from there.
I feel your pain. Been there, done that, still doing it. I won't go into details on a forum such as this though.
I think you that you are correct in that you should assume liability if it can be determined that your work caused the problem. Personally, I like the approach of "I can send Joe Contractor over to take a look and attempt to determine the cause of the leak. If it is something that our work caused then we will address it accordingly."
Be careful though because if you assume liability where you have none that buyer will call you back for every little issue he/she finds with the house going forward. Everything. They will treat their house as if it should have been brand new when they bought it. In one case I couldn't touch the issue (which the homeowner clearly caused himself) and he and his Realtor were PISSED at me. So I simply said "Here are the names and phone numbers of two local plumbers who have outstanding reviews from customers. I am sure that they would be willing to look at the problem and provide you with options on how to fix it".