How to sue a Apartment Complex?

I moved out of my apartment in 2006, and I moved into a new apartment. then I left that apartment in 2007, and i was about to move to my new apartments and the manager said i still owed money on the first apartment i lived in, in 2006. I went back to Suntree Apartments in Kansas City, and paid them the total amount due, so that i could move into my new apartment. So now in 2008 i trying to move again and Suntree says i never paid them. I sent the lawyer (Suntree's Lawyer) a copy on the money order receipt, that also showed the property manager signature on it. And i thought i was all done and over with. Then I get a notice that Suntree was sueing me for the money that i already paid them, they said that the money order was not clear enough to show the amount, so I advised him to call the money gram hotline, and it will tell them who cashed it. So Can i counter sue these people or what?