Collection agency going after more money from me

I was involved in an accident which I was admitted to a hospital and surgery. A few months later, I received a letter from the Collection Agency requesting me to pay $4800. I called them and explained to them I had insurance with my mother(which later I found out I wasn't covered, because I was out of school).

So I proceeded to contact the person's homeowner's insurance at which the accident happened. The homeowner insurance paid the debt collection agency $5000.00 (which is the limit for the insurance). Now, the agency is harrassing me to pay them more money. Their reason is being that I didn't have insurance with HMO, so I should be paying additional 3k. I was furious and didn't know what to do.

After a few battles on the phone with them, they said they would give me a discount and pay $900 to settle the debt. My question is, are they robbing me of my money? When I asked for a return envelope with their name on it in order to mail in the payment, they said they didn't have any (which is a lie because my first bill from them contained a return envelope). I spoke with the manager at the agency today and he told me $4800 is the amount I had to pay if I had insurance (EX. HMO), $8000~ is what I owed if I wasn't insured. I always thought an uninsured person would get some kind of a discount???

Anyhow, I will be contacting the hospital today to find out what they have to say.

Any sugguestions during the mean time?

FYI, the collection agency has changed their story on me a couple of times already, but they are still after me for 900 bucks.

Any help is greatly appreciated!