Praise be to Allah, the Lord of the Worlds.
The above transaction doesn`t have one specific legal framing. Rather, it goes in between the following transactions: Non-refundable deposit sale, brokerage, Ji`alah (Conditional payment), and loan. However, it doesn`t meet all the conditions of any.
A careful examination of this transaction shows that it involves uncertainty and gambling as the small trader isn`t sure whether he will collect his thousand JDs or not. Reserving the right to the big trader to sell that car, in case he felt that its market value will drop, and burdening the small trader with paying the difference is what makes this transaction involve risk and gambling for him.
In conclusion, the above transaction is prohibited because it violates the teachings of Sharia. And Allah knows best.