Two people came to me; one of them wanted to buy a car for cash and sell it to the other on credit.
They both chose the car together and I doubt that they agreed on the price on credit before purchasing it from the agency. Is it permissible for me to sell it to them?
It is permissible for you to sell it after you have taken possession of it. Doubting that it will be sold on credit has no effect on the transaction.