I've always assumed that severance pay is not an artifact that is negotiated upon severance, but rather that it comes into effect once the necessary conditions vest, so to speak. Severance pay, and an employee's right to it, is implicitly contained in the employment agreement, and once the conditions for payment laid out in the employment and labour law regime under which you were hired are met, then your right to get paid automatically comes into effect. The idea that an employer can refuse to pay you on the basis that you have refused to sign a severance agreement seems out of whack with the spirit of the law to me, assuming that this is not explicitly stated either in law or in your employment terms (that your being paid for severance is subject to you signing a severance agreement).