Company Overpaid Me For An Entire Year By Mistake And Is Now Demanding I Pay It All Back. Location: Illinois.

So I need some help understanding my options here. Location: Illinois.

I work in account managment and for the past 14 months I've been recieving an additional $440 per paycheck on top of my base salary. I assumed this was a skills supplement that had been approved - I had taken on several additional responsibilities around that time and my direct manager had mentioned something vague about "compensation adjustment" in a meeting. I was responsible for client payments (although this is a finance job) and coordinated some projects.. I never got anything in writing but the payments showed up consistently so I didn't question it.

Last week HR called me in and told me there had been a payroll error. Apparently someone entered a recurring bonus code that was meant for a different employee in a different department and it just kept running for over a year without anyone catching it. Total overpayment is around $8,700.

They handed me a repayment agreement to sign asking for full repayment within 60 days. They suggested that it be deducted from my salary or that I simply pay it myself.

After talking to the team leader and manager, I was told that my salary would be reduced to the level specified in the contract, which is fine, but I am doing extra work for other people, which is not right. I told them I needed time to think and didn't sign anything.

Here's my problem. I didn't save that money. I adjusted my budget when I started recieving it, paid down some debt, covered some medical expenses, I even traveled a little. I genuinely don't have $6,200 sitting somewhere I can just hand back.

My questions are: am I actually legally required to repay this? Does the fact that they let this run for over a year without catching it affect their claim? And if I do have to repay, can they force the 60 day timeline or can I negotiate something longer?

Maybe I can just quit, as if finding a new job with my experience will be easier, because I don't know how to work with these people after this, like it's not my fault, but the fault of a bunch of managers, executives, lawyers, financiers, etc., who sign everything and make payments, but I'm the one who has to pay.

I haven't signed the debt repayment agreement yet, so I think I need some advice...