Signed severance agreement and then company is trying to rescind saying old agreement in effect

My old company (Company ABC) filed for bankruptcy and the division I worked for was sold off to another company (Company XYZ). I was transitioned to the Company XYZ after signing an agreement that said that I would not be eligible for severance from Company XYZ and that my prior years of service at Company ABC will not be recognized until after 6 months of employment with Company XYZ. Two weeks shy of 6 months, Company XYZ laid me off. But during the exit interview, they said that I was eligible for severance and had me sign an agreement to that affect. Now, 4 weeks after I was laid off, I received an email from Company XYZ's HR saying that they made a mistake in giving me severance and that I need to pay back the severance I have already received. Do I have to give back the money? Does the new agreement they had me sign take precedence over the old one or no?