What is the ruling on the amount of money my friend paid me to purchase a car from the National Bank in my name, with him bearing all the installments and fees?

The reality of this transaction is that your friend appointed you as an agent to buy the car for him in your name from the bank, with the price being a debt. Agency in buying or selling does not require explicitly stating the principal's name; rather, the agent can buy in his own name. If the amount your friend gave you is a fee for the agency, then there is no harm. However, if it is in exchange for the guarantee, then it is not permissible, because you are, in reality, a guarantor for him, and this guarantee might lead to a loan that yields a benefit for the lender, which is Islamically prohibited. It is preferable to abstain from taking this amount.