I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today.
Yes, the handshake agreement is valid as between you and the contractor and can be sued on in court as a breach of contract of the contractor does not follow through with the agreement he made with you.
But rather than just rely on the handshake agreement, you can ask that he simply write a short note memorializing the verbal agreement, that in exchange for you signing the completion form now, that he agrees to redo 5 of your windows. Then you will have a written agreement which he will not be able to avoid.
However, even a handshake agreement is enforceable in court.
You may reply back to me using the Reply link and I will be happy to continue to assist you until I am able to address your concerns, to your satisfaction.
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I wish you and yours the best in 2016,