How do you buy a car from someone if they are still making payments on it to a dealership?
Since the person trying to sell the car still has a loan to pay off, he does not hold title to it - the lending dealer does and he will want his money right off the top. you will need to include him in the sale so he can insure his rights are upheld. I would contact the dealer and ask him how he would like to proceed.
Perform the sales transaction at their lenders office. That way the loan will be paid off and you will get the title in a week or so.