Is there a sin upon a customer service employee in a car dealership that deals 90% with financing companies and banks that fund customer transactions, knowing that their role is limited to giving the cash price of the car to the customer, without participating in installment processes?

It is not permissible for the company to deal with usurious banks and financing companies, because they do not purchase the car; rather, they pay its price on behalf of the customer with a usurious increase, which is a prohibited usurious loan. Therefore, it is not permissible to direct anyone to them, nor to allow their representatives to work permanently in the company, nor to assist in the usurious contract.

As for selling a car to a customer who borrowed money from a usurious bank, it is permissible, because he owns the money through the loan. The prohibition is in participating in the usurious contract, or directing others to it, or allowing its representatives to be present in the company. This is based on the Almighty’s saying: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression), and the Prophet's saying, peace be upon him: (The Messenger of Allah, peace be upon him, cursed the one who consumes Riba, the one who feeds it, its scribe, and its two witnesses, and he said: They are all equal).

If your work is not connected with directing customers to usurious banks, nor with writing usurious contracts, then there is no harm in it; as the general rule is the permissibility of working in companies that deal with usury, as long as the employee does not directly engage in the prohibited act nor assist in it.