Is there any thing I can do to fight National Credit Systems collection agency?

I have a ongoing issue with my account. The payment was paid in full over a month ago...when I made my last payment I was told by the representative Diana Abbensett that I owe an additional 30 for a NSF charge. I informed her that there was no reason for there to be an NSF charge. There was suppose to be two drafts taken out of my account one on 12/31/10 and 01/14/11. I called soon after the 01/01/11 informing Diana that the draft did not go through on 12/31/11, she informed to give it some time b/c due to the holidays it might be delayed and other customers were having the same issue. On 01/13/11 Diana called me and informed me that the draft did not go through. I informed her that it did not post to my account and I ask her did she have the right information and she stated that it was correct. She proceeded to ask me to make a payment, I paid with the debt car attached to the account. I ask her could I just make the other payment at that time she stated since the other payment had not went through to the account yet, that there was a possibility that it could post to my account correctly. Then I turn around and made the other payment. She informed me that in order to prove that I did not owe the charge she would have to have a copy on my account statement showing that at any time during the time period of the payments that money was available to process both payments. I fax her the documents and she informed me then it would take more time to view the documents. I called her back and she read off my information from the documents and informed me again that she did not know what happen and that everything should be taken care of. I informed her that I was in a hurry because I am trying to move. It wasn't until I called my old apartment complex and the informed me that they were not given any notification stating that the debt has been paid. I called Diana and she told me to get her the fax number as if she was going to give them the information that was Tuesday of this week. I called today to the new apartment complex where I am applying to and they informed me that there is 30 dollars still showing that I owe on my credit. So I called Diana back, so she states that now she is unable to see my faxed documents, I ask her why she stated that she did not know, and that I needed to prove again that money was in my bank account 12/31/11. This upset me because now we are back to square one, I told her that I already sent that information over and why doesn't she have it, she said she didn't know. I asked her to put someone on the phone that did know and that could get to the bottom of this, so she put a gentlemen on the phone I did not catch his name. He proceed to tell me to fax the documents again, then he stated to just pay the 30 dollars, and he stated it was because I closed my bank account and then reopened it. Their website is reflecting that I only paid 610 and I paid 640 and I have a letters from them proving it.