Sixt — Refund owed

I was the unauthorized driver of a sixt rental car. I wrecked the car and had to pay for the sixt car and the other vehicle. I took out a loan and sent sixt the total price of 23000 dollars through the post office with signature required. It was traced to the sixt mailbox. But sixt said they did not have the check and it was going to be sent to collections. The registered check could not be refunded from the bank could not be refunded for 69 days. I then set up a payment plan of 3900 dollars a month and paid 2 payments. Then sixt found the check fir 23000 and the debt was paid in full they now owe me 7400 dollars. I have sent email and multiple calls but I am unable to get my refund. So I am paying two loan payments for the same debt while out of work sick Send refund of 7400 dollars which is owed