Product: Debt collection

On XX/XX/XXXX, while vacationing in XXXX XXXX XXXX XXXX, my wife and I upgraded our existing XXXX timeshare through XXXX XXXX XXXX XXXX de XXXX. At that time, the total selling price was listed as {$21000.00}. We were credited with {$18000.00} of equity from our fully paid original timeshare with the same company. After paying down a portion of the balance, we financed the remaining balance of {$3700.00} through a promissory note carried by the same company. Less than a year later, we paid this financed amount off in full and were sent a " paid in full '' document dated XX/XX/XXXX. 
In spite of minimally using the timeshare, every invoice for maintenance fees was paid when received and this continued through XXXX. In XXXX, we relocated from XXXX   state to XXXX  and filed a change of address through the XXXX XXXX XXXX. We never received an invoice for maintenance fees after that. Knowing that XXXX timeshares are leased for a set number of years and not sold outright, we assumed the lease period had expired. 
In XXXX, XXXX, my wife and my previously unblemished credit showed a serious derogatory due to an unpaid collection from Monterey Collection Services ( MCS ). I contacted MCS to get something in writing from them about an alleged opening balance owed of {$2500.00}. In a letter dated XX/XX/XXXX, a XXXX XXXX Representative ( XXXX XXXX ) sent a packet of about 20 sheets which were from XX/XX/XXXX. There was no explanation, so the implication was that the amount being collected had to do with the purchase of the timeshare upgrade on that date. Knowing that the promissory note related to the XX/XX/XXXX transaction had been paid in full, I was confused. I called XXXX and was then told that the collection was related to unpaid maintenance fees. I then requested something in writing so I could verify the amount allegedly owed. 
On XX/XX/XXXX, MCS Collections Representative ( XXXX XXXX  ) emailed me a 23 page detailed ledger covering the 13 year period from XX/XX/XXXX to XX/XX/XXXX. This ledger shows that maintenance fees were paid through the time we moved. However, and most importantly, the ledger balance NEVER showed a balance of the alleged opening balance of {$2500.00} claimed by MCS. MCS continues to claim this original balance, then has added on various late fees and interest to derive what they claim is the current balance on this account. Without being able to verify the claimed amount as an original balance, it is impossible to access if the claimed amount owed is accurate. 
Even if there was some indeterminate amount of unpaid maintenance fees owed, the ledger received shows that the timeshare was foreclosed upon. According to the timeshare owner 's own records, the timeshare was worth over {$21000.00} in XXXX. If the timeshare was foreclosed, it was then resold and my wife and I have no rights to it. Upon resale, the timeshare owner XXXX MCS 's client ) received many times any claimed unpaid maintenance fees. To then extract more money from us on top of what they already received by reselling the timeshare would result in MCS 's client being unduly enriched. This is grossly unfair. 
I have tried to lay these facts out to XXXX  and disputed this collection on three separate requests. Unfortunately, XXXX  has concluded on each of these three occasions that I am liable for the monthly rising balance that MCS is claiming we owe. This position defies logic. Because of the inability of MCS to document how the alleged opening balance of {$2500.00} was derived, the only equitable thing is to remove this collection from both my wife and my credit report.