I appreciate all the responses. I am a little late, but here is the update. I wrote the following message to the buyer". Prior to responding to him, I checked my contract and consulted my lawyer to make sure I'm in clear.
"Sir, in regards to your message it's very unfortunate that this is happening to you guys; however, there is nothing I can do. I would advise that you go through your home warranty or homeowners insurance. In addition, I would recommend getting more quotes as these numbers sound excessively high. Again, it's unfortunate circumstances and I wish you best of luck".
His response was-
" I went through the home owners insurance which is AHS, and they won't cover the issue with the plumbing. Charged me $75 service fee and didn't do anything except agree with the other plumber that the main sewer line is collapsed. Roto rooter (a local plumbing company)cost me $519 that American home shield won't reimburse me".
My response to that was...
"While the property was under my possession everything was functioning properly. We had no issues with plumbing at all. Also, an inspection was done prior to closing and everything was fine. These things occur unfortunately and they come with territory of being a homeowner. Again, I'm sincerely sorry that this is happening to you. There was no intent to do anything that is unethical. I take a lot of pride at my work and always address the problems as they present themselves before me".
After my response, he did not reply anymore. Again, my conscious Is clean. Things happened and we just have to deal with it. If this would have occurred within 24 hours, I would have done a different approach, but a month later is not reasonable to me when the house was sold AS IS. Anyways, thx everyone one more time. Merry Christmas and Happy Holidays!