company trying to back out of part of severance agreement

I recently signed a severance agreement that, in addition to 17 week's salary continuation, also provided for me to retain my laptop for the same period of time since I don't currently own a PC and since I'm unemployed, do not feel that now is the best time to spend the money to buy one. It does not say that I'll have to give it back early if the company decides they need it. This means that if necessary, I should be able to keep the laptop until the middle of November.

I have now been contacted and told that my laptop is needed elsewhere and that I must return it by September 1. When I pointed out that my agreement says otherwise the somewhat threatening response was "you want to renegotiate?" My question is, once an agreement has been signed by both parties, can they renege on a portion of it and/or declare the entire agreement to be null and void?