Severance is as per an agreement (even if there's no written agreement, it's still as per an agreement--simply an oral or verbal one). If the terms  of the agreement had been that your husband would return severance if rehired, he'd have to return it. Or if they offered him a job only on the previso or condition he returned severance and he agreed to it, again, he'd have to return it. But if the severance agreement never required its return, and the job offer did not have that string attached at the time your husband accepted it, then they cannot require him to give back the severance--one party cannot add additional terms or conditions to an agreement after the fact. Now, if the company asks for the money back, if there had been no previous agreement, it may be a good idea to return it--but that's for you to decide, weighing factors such as the amount of money and the value of good relations with his employer.