I would think carefully about the matter of a lawsuit (or even just the threat of one). Essentially, the horse has already bolted when such an unfavorable contract was signed. Even if the flat rate is only fixed for a limited period, there should be something in the contract about an alternative ending? As it stands, it is simply open-ended. He could now demand 200k. Where is the limit of good taste, what is reasonable, what is legitimate? I find the wording itself more than dubious.
But be careful with a lawsuit against a construction company. They may have the stronger hand, also financially. Such lawsuits quickly become very expensive. Every legal expenses insurance excludes disputes over real estate for good reason. No one covers these costs except yourself, and you would have to come out of a lawsuit as a winner not to be stuck with your costs. In a settlement, each side already pays its own legal costs, and that quickly goes into amounts for which you would have to take out a new loan.