Non-Disparagement after I gave notice?

I am being required to sign a non-disparagement agreement from an internet start up company as a pre-requisite to getting my last salary check and expenses that are owed to me. I did not sign a letter of employment with this company as I disagreed with some of the verbiage but they never changed the letter of employment nor did I ever sign one. Now, as a condition of my resignation they are forcing me to sign non-disparagement. Can they do this and hold up my earnings and expenses that have already been approved? They are not paying any other monies to me for stock options, vacation, etc. I am just looking to get my final salary check and expenses back.

Thank You...

[judg4]

No--you may sue in small claims if need be. You have a right to the paycheck.

So, the paycheck I can uderstand, but what about expenses owed for the last 3 months?