Product: Vehicle loan or lease

My son purchased a Vehicle in or around XX/XX/XXXX. The vehicle was a XX/XX/XXXX XXXX XXXX, which was financed with Santander. We received our first statement dated XX/XX/XXXX, showing a payoff of {$14000.00}. At this point, no payment has been made, the statement was for the 1st payment which had a due date of XX/XX/XXXX. 
I was making my sons car payment, he had become unemployed after we purchased him the vehicle. I made the payment of {$380.00} that was due for the XX/XX/XXXX payment. 
On XX/XX/XXXX my son was in an accident and the vehicle was determined to be a total loss. I contacted Santander and advised them on XX/XX/XXXX that the vehicle was a total loss and inquired if I should make the XX/XX/XXXXpayment or not. I inquired as, we had full coverage and GAP insurance, at that point I knew the vehicle would be paid in full regardless of my making the XX/XX/XXXXpayment. I was advised by Santander not to make an additional payment. 
My insurance company issued a check to Santander on XX/XX/XXXX in the amount of {$13000.00} ck no. XXXX payable to Santander total loss. The insurance check plus my XX/XX/XXXX payment was enough to pay the loan due in full. While I was reviewing my documents to cancel the GAP insurance, GAP was never required since the insurance paid the loan in full, I realized that Santander received an additional payment from my insurance. On XX/XX/XXXX, {$1100.00} was issued on ck no. XXXX to Santander. 
Today, XX/XX/XXXX, I called Santander to ask why they collected, in total {$15000.00} on a vehicle that had a payoff of {$14000.00}. I received a payment detail report, which was intended to explain this to me however, the report also showed an amount due of {$13000.00} with various payments applied and reversed that didn't match the actual payment records. As of today I have spoken with three ( 3 ) people about this. XXXX ( customer service rep ), her manager XXXX XXXX, and another CSR named XXXX. None of these reps could explain how they applied these funds to the payoff and how the payoff exceeded the contractual amount. I understand accrued interest, CA is a fee simple state and I'm aware of the per diem. The only explanation I received was the additional payoff amount was for accrued interest. 
By the time I spoke to XXXX, he looked " further '' into the account and explained that what had happened is : An adjustment was made in their system to reverse the XX/XX/XXXX payment I made, this was done on the grounds that payments aren't to be posted on vehicles in a " total loss '' state. Still don't understand that since it was for a XX/XX/XXXX payment, due before the loss?? So, a rep in Santander went into the accounts records and manually reversed a payment that they had in fact received and cleared. Upon doing that, it now showed that the account was not satisfied with the payoff initially received from my insurance in the amount of {$13000.00}. XXXX then explained that as a result, they accrued interest from XX/XX/XXXX through XX/XX/XXXX when they requested an additional {$1100.00} to satisfy the account. 
Now that to me is a clear case of falsifying account records. Santander was paid in full with the initial payoff. I was never credited back the {$380.00} that was " reversed '' from my account and, The {$1100.00} was erroneously accrued on account of the " reversal ''. To date, I've received no credit back from Santander, not for my payment that they " reversed '', not for over collecting on a payoff, nothing. They have also refused to provide me a break down of the payoff so I may review it for inaccuracy. I've not been allowed to speak with a Payoff department, or anyone that can field my complaint about falsifying financial records. I'm asking for your assistance in highlighting this significant short coming in financial accuracy and responsibility as well, to recover the funds that were inappropriately collected by Santander.