Early this year, I was laid-off and I was sent a "SEPARATION AND RELEASE AGREEMENT" which I signed because I did not have any choice. I was told that if I signed it I did not have to paid the balance for a sign in bonus. This week, I got a letter from the same employer stating that there has been a mistake and I need to pay the balance from that sign in bonus back. Also, they are threating me that I will have to pay for lawyer, filling and other expenses that they might incurr.

Obviously, I do have copies of all the e-mails and documentations to prove and backup that the bonus have been forgiving.

Is this legal? Can I sue them from breach of a contract and punitive damages?