<prompt> Human: I boughtfinanced a used car about a month ago. I made a 4K downpayment the day when I purchased the vehicle, but because I wasnt planning on buying the car that day I didnt bring in my old car for trade-in. My dealer gave me 6k for my old car without taking a look at it because I provided my old cars insurance with VIN info. However when they pulled my credit and got me a finance, the agreement loan amount didnt include the 6k trade-in value which was understandable because I didnt bring my car in yet. I brought my car in two days later and signed a bunch of paperwork, and on the spot I made sure to ask the finance guy that this 6k is going into my principle that day. He said yes. However I didnt get any form or copy of confirmation, because the dealer guy said I would receive a packet in the mail. I received my finance packet from the bank yesterday and the 6k was not applied to the principle, and its been a month since I brought my car in when they were supposed to do so. I will be contacting the dealership today to see how theyve made this mistake on their side, but was wondering how should I investigate the process and the extra interest that has already been added to my principle for the past month because of the extra 6k. Right now I do not have any form of proof that I traded in my car for 6k, but the dealer has my signed title for transfer for my old car. Any helpsuggestion is appreciated. Thank you!UPDATE: So the finance department investigated and apparently, they simply FORGOT ABOUT MY TRADE IN VEHICLE... The manager offed me a 6k check and will arrive in mail in 7-10 days past my first payment due date. I would definitely like to get compensated since this is a complete mistake on their side. Besides mentioning that I paid extra for Sales Tax and interest, is there anything else I can use as a leverage to negotiate? Appreciate all of your help and thank you!  <response>: I sign nothing unless I am okay with the deal. Im not taking a finance salesmans word for it. Always get copies and have the paperwork. You dont even have to ask the finance guy, because you have it right there in writing. 