The question

Hi, I was with Employer1 till Nov1st 2021 and they let me go giving performance reasons but were also kind enough to give a severance pay of $x for working there for 3.5 years. I started working in another job - Employer 2, on Dec6th 2021 and am continuing to work there. The severance was paid on Feb 4th 2022 and now in Dec 2022, Employer 1 is coming back and asking me to pay back the entire severance, which I don't understand at all. Is this even valid? I live in California on H1B Visa.