Fard One Inc created extra charge on my credit card. Chicago Illinois

I was a customer of FARD ONE on the 2nd of September, 2014.....and I was falsely gouged for 20 dollars.
On Sept 2, 2014 I stopped at the Shell Gas station(FARD ONE Inc) on 1721 S. Paulina, here in Chicago, to fill up my "motorcycle" tank with gas. When the card did not work at the pump I walked in to the small building where the cashier was behind the window. I told her I wanted to buy gas but the keypad on the pump was not working. She told me no problem and swiped my card and charged me $20.00. I asked her why did she do that? I haven't purchased any gas and secondly my motorcycle could never hold that much gas. A man who appeared to be the manager standing over her shoulder ordered her to refund me the money and then he promptly walked out. She said she would while I pumped my gas and to come back when I was finished. I came back and she handed me a receipt for "0" credit claiming she cleared the 20 dollars and then proceeded to charge me again on my Mastercard for the $11.45 actual gas bill. I asked again about the original 20 dollars and the cashier told me she cleared it and it was gone. I left feeling cynical and sure enough the next day (Sept 3, 2014) I looked online to find she had charged me for 20 and the legitimate charge of 11.45. Disappointed, I called my Citi Mastercard and reported the fraudulent charge and suggested they see if other card members had been double charged by this Shell franchise. I also called the only number I could find on the receipt (1-888-98-SHELL) that turned out to be a number to apply for a SHELL CREDIT CARD. When I reported the Shell station and the details to the representative she told me she would transfer me to a customer relations representative. This representative named "Eric," then took down all the information and stated he would have someone call me the next day after 3PM. It is now 8:17PM and no one called. I just checked again this evening and the extra charge for 20 dollars remains on my Mastercard.