Car detailing service near Gaithersburg, MD

The price of the vehicle I purchased changed (increased $300.42 by his wife ) from the agreed upon original vehicle price I made with Mr. M--- after I signed and closed the bill of sale with him. This increase affected the amount financed. When I brought this matter to the attention of Salim (his wife) , she stated that the finance company was charging me the $300.42 for not paying cash and because they were financing the balance of the sell for me. I contacted the financial institution WL and the representative stated that their company does not charge such a fee. The finance company was concerned that bogus information was being given to customer. I immediate called the salesman and his wife. I left a message for the wife. I spoke to M---- who stated that this fee was the Ameri-Benz doc fee. I told him I had already been billed a doc fee of $29.00. He told me to contact his wife, who will not return my calls. I have been calling the salesman and his wife since 3/2/2016 (made my concern known on 3/1/16) to date. I have called the dealership twice to try to resolve this matter to obtain a refund for the erroneous fees she assessed me. I have placed 7 telephone calls collectively to the owner and to his wife (who prepared the loan papers) and to date neither of them will return my telephone call.