Is it permissible to work as an accountant in a manufacturing company whose capital came from selling cars purchased through an installment system with interest, knowing that the nature of my work will involve calculating the value of the remaining installments for the lender?

There is no objection to working for the aforementioned company if the work itself is permissible, even if there is an element of doubt or unlawfulness in the company's capital; the sin for that rests with them. The method mentioned for their installment dealings with the car owner is permissible as long as he owns the cars and sells them for a known price, even if the price is higher than the market value, because time has a share in the price.