USCB Corporation Took my money and now they want more! Oaks PA

 I attended Penn Foster College but times got tough and sadly and unfortunately had to stop. I ended up having an $816 remaining balance that had to be unpaid for a while. My account was then placed in collection with USCB (which I knew was going to happen because the account was unpaid). However the company ended up calling me 8-10 times a day and sent letters at least once a week. Eventually I got a letter in the mail stating that I could pay $407.95 and my account would be good and taken off my credit report.

So I did just that, I paid that amount and as I was getting off of the phone the lady had stated that I could now return back to school if I wished. I asked her if I could give it some thought and she said "of course!" Well a few months pass and I decide that I really need to finish my degree and try to re-enroll at Penn foster and they blocked my account, saying I needed to contact USCB to fix the situation. I called and get told that I still need to pay the remaining $407.95 before I can go back. I told them that I did the "deal" they sent me stating if I paid the discounted price, that my account would be in good standing. Now I am back to getting threatening calls and letters in the mail. What do I do??