Product: Debt collection

The co tenant and I had to opt for a contract Buy-Out option to end our lease early at the XXXX XXXX XXXX XXXX locaXXXXed at XXXX XXXX XXXX XXXX , XXXX XXXX , CO XXXX . We submitted, by email, the required 60 day notice on XX/XX/XXXX, which went unanswered. I submitted a ticket through the XXXX XXXX XXXX, portal on XX/XX/XXXX, requesting acknowledgement of receipt, also unanswered. I then sent a follow-up email on XX/XX/XXXX, requesting acknowledgement of receipt, of our 60 day notice, this was also unanswered. I no longer have access to the apartment portal, but I have the ticket number. 
On the day of our move, XX/XX/XXXX, we hand delivered the required amount of {$3900.00}, of which I have a copy of the cashiers check signed by an office staff employee, the total of two months rent. At this time we inquired if anything else was required, we were told that nothing else was needed. 
We did not receive any written communication regarding any money due as the collection agency was sending all communications to our previous XXXX XXXX address, although our new address was included in our 60 day notice, and also left with the office staff member at the time the keys were delivered. Due to an issue with the U.S Postal service we were not able to immediately proceed with forwarding our address, meaning we received nothing to allow time to dispute the issue. I finally received one phone call from IQ Data, the collection agency hired by XXXX XXXX, on XX/XX/XXXX, and was spoken to in an abusive and harassing manner. When I got nervous and confused I was told to act like an adult by the representative. She, XXXX, then threatened if the payment wasn't made during the call that she would report the debt to the bureaus after our call. I asked for time to look into the matter and she said there was no other option, pay now or it goes to the bureaus. 
Once the postal service allowed our address forwarding, we finally received the written notice stating that we had until XX/XX/XXXX to dispute. I was able to submit before the deadline, by email, on XX/XX/XXXX. It was shortly after that I got a notification that I had a collection on my credit report, IQ Data had already reported to the bureaus. This debt was paid in full. The apartment management is stating that we left early, so damages are due, however, we followed all protocol for the contract Buy-Out option. It was finally acknowledged by the apartment manager, on XX/XX/XXXX, that we did indeed send our 60 notice. It is nearly impossible to reach anyone by phone. No emails are acknowledged unless you are lucky enough to reach someone by phone and wait for them to open their email to locate your message. Before move in we paid first month, and a full month deposit, on top of additional deposit monies and we were current on our rent. We had no choice, we had to vacate early due to a job loss. We left on good terms, paid in full, in compliance with the contract Buy-Out agreement. Now they will not recognize their own contract, they have negatively impacted our credit and will not provide a paid in full letter and receipt and they still say we owe money. At this point I am concerned about the validity of this entire establishment.