I was hired on as a full-time salaried employee ($57.5k) and fired on the morning of my 3rd day. I received an email confirming my termination date and told that I would be paid for the 3 days via payroll.

Several days later, they emailed back asking me to submit an invoice for days worked. Having worked as an independent contractor for my entire career, this signaled to me that they wanted to file me away as an independent contractor, which I've read online is illegal, but I'm fine to be done with this ASAP. So I sent over an invoice with 3 days billed at my regular daily rate of $500/day.

They've now responded by saying that's the incorrect amount and that I need to go by the salaried amount which results in $221.15/day, but can be submitted as $250/day, so $750 total.

My question is what is the best way to proceed? I see no other reason for them to ask for invoicing aside from wanting to file me away as an independent contractor, thus, in my mind, nullifying the entire contract. I also read online that it's not only illegal for them to misclassify me after the fact, but that it's also illegal for them to withhold payment pending an invoice since I was hired as a salaried employee.

What is the best way to respond so that I'm not aiding in their illegal activity of misclassifying me, and so that I don't receive further communication or instruction from them, as I no longer work for them?