apt charge 1200 after move out

I moved out of an apartment this end of June and just found out on my credit report that they charged me 1200 dollars for : Cleaning, painting walls, Carpeting, change locks and etc, I am so pissed off. The collection agent I just talked to said they have sent me letters in Aug, and Nov for me to dispute, but I never received any mails from them. Now they offer me to settle it before this month at $980, or return the case to the apt so that they can file a charge on me. (wuwuwuwu). ......What should I do? Da Xia please help!!!!!