Employment Law Questions? Ask an Employment Lawyer.
John, from what you have put out here I don't believe they can withhold payment for work you have done. It would be illegal for them to withhold pay on the basis that they are trying to strong arm you into an agreement. This unless they have given you severance as part of a speartyion agreement from your company. Then that would be something they could ask for. They could have asked you to sign the confidentially agreement prior to your deciding to leave
or at the beginning of your employment. At any rate tour pay check is for work you have performed already, not a quid pro quo for you acceding to their agreement. If you have more details for me to consider please let me know. Best, Sean