Help Please on the realty dispute !! 單身老貓 , Expert!

We sold our old house on Aug 5, 2019. We bought that house in 2011. We then added 
an addition (the original builder of the house) and the pool in between 2015-2016.

We lived in that house more than eight years. We love this house.  The reason we sold this house was my husband changed job to different city. This house is perfect. The only one small events happened was several months after we moved in 2011. One day We found water came up of the bath tube. We then called plumbing service, they came and did the service, the issue was resolved. Ever since then the house had no issue. The house was in excellent condition when we sold it. Passed the buyer's inspection.

What should I do? How to deal with such bully? The money they demanding is scary!

Please help.

Thanks.

Queen

   ==================Letter received today from buyer's lawyer==============