S.,
I am in the mortgage business (going on 9 yrs now) and deal with this kind of thing all the time.
First, get a copy of your report from all 3 repositories (typically called a Tri-merged report): Experian, Transunion, and Equifax - you can get it online at Freecreditreport.com, or I highly recommend Suze Orman's site: myfico.com, OR you can call your mortgage lender and they'll likely pull it for you and send it to you via email.
Second, write a letter to each of the repositories reporting the debt (sometimes it's just 1) and send the copy of the letter from the apartment. In your letter you ONLY need to say, "this information is inaccurate, please update the records". That's all! they don't need additional information to start their process.
Third, the repositories will initiate an investigation to the apartment or collector and if they do not respond within 30 days (or if the information does not match), the repositories are required by law to remove the debt.
Let me know if this helps or if you have additional questions. ____@____.com
C.