I went through similar thing years ago. The seller/builder was about to lose the place in a tax sale. He is trying to avoid paying the taxes because he knows they will snatch what he owes at the closing table. If you can risk it and there is no back up offer, threaten to leave the deal unless they close in a certain time frame. His taxes aren’t your problem and make sure you have it in writing that they are going to pay for your interest rate lock. Btw who is the builder?