What do I do if a company is asking to pay back the money from full and final settlement received months back?

My previous company paid Full and Final Settlement in October 2022, now they want half of it returned. They sent me a email on April 21 2023. 1. How can they say it was extra payment, after 6 months? 2. They threaten to send me notice, what are my legal options? 3. I lost that money in the stock market, and will take 5-6 months to get that amount back, how long can i delay payment, if there is no other way? 4. I already paid income tax on it, now what should I do?