I paid Sallie Mae $500

I paid Sallie Mae $500 on 12/21/12, which covers my monthly payment of about $493. On 12/27/12 Sallie Mae took an additional $493 claiming i was signed up for automatic withdraw even though i never signed up for it and they have never taken money from me automatically before. I called them and they said i had to fax them my bank statement to them showing a double payment, i did but they never called me back. I called again the next day and they admitted their mistake and canceled the automatic withdraw and they would refund my money. After waiting 10 days they finally called me back today saying my claim was denied because it would make my sallie mae account go into withdraw, which is not true because i have already paid $500 for the month of the december. This has caused me to also fall behind on some of my other bills and my bank charged me for overdraft because of the $493 they took and i did not have the funds to cover it. All im asking for is the $493 back they took on 12/27 without my permission. Thanks