Can i be force to pay a balance that should have never occured and that i never agreed to?

I signed up for Aetna insurance through my company. Money was never taken from my check. After further inquiry i found out that this was because no one was hired to do the accounting in that aspect for the department that i worked. Over the course of a year i went to to the dentist twice and my son who was also on my insurance continued to use govermental aid via medical card. Being that i not only never recieved a card but more importantly never paid a dime after 6 months I was under the impression that my company had not " worked out the kinks" and that was not covered. A year or so later they have come to me and said that they have paid 4,176 dollars on my behalf and that i must repay them or I am fired ( not scheduled till i can pay the balance in full). Other departments in the hotel did not have this issue as they had a accountant to resolve it as it occured. I feel like I am being strong armed and taken advantage of .Please help is there anything i can do? Is this legal?