The language of the contract can be very important, such as whether it includes a disciplinary process, whether it restricts the employer to terminating "for cause", etc. I suspect that your employer feels that they cannot legally fire you without paying you something, as they are making the offer. At this point your most obvious options are to (a) accept their offer, (b) make a counter-offer asking for a larger buyout, or (c) decline the offer and decline to quit, to force them to either continue your employment or breach the contract by firing you anyway. A consultation with a lawyer is a good idea, particularly if you're thinking about option (c).