What is the ruling of Sharia on a Muslim helping his customers obtain usurious loans from financial institutions to purchase cars from his showroom, knowing that he only receives 75-90% of the car's price from them or from the institution?

This method is Islamically forbidden because it is pure usury (riba). Dealing with a usurious institution that pays the value to the seller and recovers it with an increase from the buyer is considered a loan with interest, which is usury of excess (riba al-fadl) and usury of pre-Islamic times (riba al-Jahiliyyah), both forbidden by Allah and His Messenger. Furthermore, stipulating an increase upon delay in repayment is also usury. Aiding usury and facilitating it for people is Islamically forbidden. Therefore, we must not cooperate in sin and aggression. Thus, it is not permissible for you to participate in this usurious transaction; rather, you must seek permissible methods.