Is it obligatory to pay the full remaining amount for the car to terminate the contract immediately, knowing that the contract was made before the company purchased the car and includes a discount for early settlement, or is it permissible to continue paying the installments if there is a need for money for other expenses like Hajj?

It is forbidden to sell what one does not own, based on the saying of the Prophet, peace and blessings be upon him, to Hakim ibn Hizam: "Do not sell what is not with you."

If a contract is made for a specific car before the company owns it, the contract is void because it is a sale of what one does not own.

It is obligatory to annul this void contract. If annulment is not possible, it is permissible to keep the car with repentance.

As for expediting the payment of installments in exchange for waiving some of them (pay quickly and receive a discount), it is permissible if it was not agreed upon beforehand in the contract.