Product: Vehicle loan or lease

I took out a 39 month lease with Ally Financial in XXXX of XXXX. I made on time payments every month. As the lease was coming to an end I wasn't sure if I wanted to keep the vehicle or return it and lease/finance another vehicle so I reached out to Ally in XXXX of this year ( XXXX ) when my lease was up to inquire about a lease extension. I was told by the representative that I can take out a 3 month extension for the same amount, but it would be month to month meaning I could return the vehicle at any time during those 3 months. That was exactly what I needed so I proceeded with the extension. The monthly payment was due on the XXXX of every month. 
On XX/XX/XXXX I called Ally to let them know that I was planning on returning the vehicle early, but wanted to replace the tires myself so that I wouldn't get charged for the wear during the lease end inspection. But I was also needing to go out of town and wouldn't be able to return the vehicle until XXXX or XXXX of XXXX, so I wanted to make sure that I would be able to do so without penalty as I was trying to avoid paying the last month. The representative that I spoke to ensured me that I had a grace period of 10 days to return the vehicle without needing to make the payment. So I went on my trip, replaced all 4 tires and returned the vehicle to the authorized dealership on XX/XX/XXXX. Upon leaving the dealership, I called Ally Financial to inform them that I had returned the vehicle and was told that I owed the final month payment for XXXX. I asked to speak to a supervisor and explained what the previous representative advised me. I was told that they would open a case so that someone from the management team could listen to the recorded call that I had when I was advised that I had 10 days to return the vehicle before the payment would be due, and that I would receive a call within 3-4 business days. 
A week later I hadn't received a call so I called back and was told that there still wasn't a resolution, but that I would receive a call by the following week. 
That next week, XX/XX/XXXX I called back checking on the status and I spoke to another supervisor who basically told me that they made a decision on my case and that I was responsible for the full amount of the last month even though they received the vehicle back within the 10 day grace period. The supervisor would not tell me if anyone had listened to the recorded call where I was advised otherwise. This supervisor also refused to add any notes to my case or request for anyone else to speak to me. He was extremely rude and just continued to repeat that I was responsible for payment. 
While still trying to resolve this, I decided to at least pay the disposition fee which I know I am responsible for, but my payment bank account was removed from their system and the system will not work for me to add it back. I have contacted them twice to try to resolve that and also have not been able to get that resolved. 
I am very disappointed by the level of customer service that I've received from Ally Financial. I have leased 2 XXXX and both times the dealership contracted me with Ally Financial and both times I have had a terrible experience. This has been by far the worst. 
I am now stuck in a situation where Ally is insisting that I pay them an additional {$510.00} for a vehicle that I don't have anymore and that I returned during the time frame that I was advised, by one of their representatives. I would appreciate any help to get this resolved. Thank you.