What is the ruling of Islamic law on purchasing a car in installments if, after one year, the buyer discovers that this transaction is usurious, yet he is unable to pay the remaining installments in a single lump sum and cannot do without the car?

Since you proceeded with the matter out of ignorance and did not neglect to inquire, there is no sin upon you. You are not required to sell the car, and it is permissible for you to benefit from it and pay its installments. Regret and the firm intention not to return to such a thing again are sufficient for you.