Negotiate my severance agreement.

I have asked to be paid for my commissions earned. We are paid on a monthly bases on annual agreements that were signed. Additionally I have asked for an additional month due the the abrupt termination. NO notice never saw it coming. They did offer me a 3 month severance. By asking for the additional money could an employer resend the severance all together? I would hope we would be able to negotiate the final number. I do believe my termination may have have some recourse if I wanted to pursue some legal action. I don't want to go there as it seems it would result in a lose lose proposition. Any insight on how these agreements can be negotiated is most appreciated.