Previous Employer Violated Fmla And Now Wants Back Pay For Insurance, What Options Do I Have?

For context, my previous employer offers no parental leave, so when my wife got pregnant, we decided I would take FMLA to bond with my newborn. I informed HR and my supervisor of this several months before my wife’s due date. My supervisor started treating me differently and pressuring me not to take FMLA. While I was out on leave, my supervisor informed me via text that I no longer had my team leader position, that he gave it to someone else. My pay did not change, but my position and responsibilities did. The new team leader told me I would now be his “number 3” guy. Prior to all of this, I have had a good relationship with my supervisor. I got exceeds expectations on every part of my annual review just before informing them about FMLA. The only thing my supervisor wanted me to improve on was working more voluntary overtime (we have talked multiple times about this, I have no desire to work overtime unless absolutely necessary as I am starting a new family and he has previously stated being fine with this).

All of this led me to find a new job while out on leave. So when I returned from my FMLA leave, I let my supervisor and plant manager know that I was giving my two weeks notice and would no longer be working there. 30 minutes later, my supervisor informed me that my services were no longer needed and to pack my stuff up and leave.

I realized after I left that they did not ask for my key fob back. So I asked my supervisor if I needed to turn it back in. He said I needed to do it by the end of the day because they would charge me for it. I told him I wasn’t coming back up there that day.

Now he and HR have contacted me saying that I owe a small amount of money for back pay from insurance they paid while I was on leave. They would have deducted it from my paycheck, but there wasn’t enough since they sent me home the day I put my 2 weeks notice in.

My 2 questions are 1) do I have to pay them back? What can they do if I don’t? and 2) do I have legal options for the FMLA thing? Is it even worth it to try to pursue something?

I’m in SC if that matters.

TLDR; my previous employer intimidated me about FMLA, gave my position away, then immediately let me go when I informed them of my resignation, and now is demanding I give them back pay for insurance. What should I do?

Edit to add: the insurance they are asking for back pay for is supplemental (Aflac, voluntary life, etc.) I did not have healthcare insurance through them.