If your bank has a record of this transaction taking place, get your bank to talk to the bank that's carrying your loan. This is why banks keep paperwork... to make sure transactions get handled properly and to back themselves up if someone screws up. Your bank is going to have a lot more clout than you walking in and saying "I paid you $300. Credit me." If your bank won't do this... switch banks... this should be an expected part of *good* customer service. If all else fails (and this should be a last resort)... think about small claims court.
