Unless you agreed to do something in exchange for the severance--for example, signed an agreement giving up your right to sue--the promise of severance is not enforceable: the law does not require severance, and it also does not enforce "gratuitious promises"--or promises made freely by one party without the other party doing, paying, or giving up anything in return. Employers don't have to provide severance (unless some contract requires them to do so), and can promise it then change or reneg on their promise.If you did sign some sort of severance agreement, however, in which you agreed to do or not do something in exchange for the money, that agreement should be as enforceable as any contract.