Cash as in cold cash? Not a check? If you used plain old cold cash, how did you transport the money, suit cases? If so, there should be some strong evidence in the dealers security cameras or something.
Either way Yikes, is all I have to say. If it was a check, it should be clear as day in your bank account of when or if anyone has cashed a check, if they lost the check, have the lost check canceled (via check#, etc) and re-write the check.
If you are saying that, you gave them 20k, and now 20k is gone, well that is a HUGE deal, that is get the police involved kind of a deal, that is hire a detective, and bring up a law suit for LOTS OF MONEY, kind of issue. If somebody walks into a dealership with $20,000, there is going to be a lot of buzz, a lot of counting and a lot of talk. The dealer can't simply make everyone disappear, or even the money...
The more I think about this, the more it sounds like fraud. As in, you are doing something fishy and trying to make 20k appear.