I am not an attorney. Attorneys are required to give legal advise. The following should not be considered legal advsie.
You are in NY.
In NY attorneys are involved in real estate sales.
Your question suggests the attorney allowed an invalid contract to be executed.
The attorney is also responsible for overseeing the transfer process.
Whose name appears on the property title?
And, from your question it appears ALL signatures were on the contract and the buyer paid up.
WHENhe buyer signed will be difficult for you to prove.
What is your REAL intention? Are you hoping to reclaim the property after receiving payment in full?
Was this a Do It Yourself (DIY) real estate transaction?