All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
Knowing that the buyer will pay the price of the car through a bank which deals with Ribaa (interest and/ or usury) and negotiating with him on the interest rate, is a clear help in the process of Riba, and Allaah prohibited this as He Says (what means): {… do not help one another in sin and transgression.}[Quran 5:2]. The prohibition is not restricted to writing the contract of Riba or being one of the witnesses in the contract rather any kind of help in the process in any form is forbidden.
When interpreting the saying of the Prophet : "Allaah has cursed the one who consumes Ribaa (i.e. usury or interest), the one who gives it to others, the one who writes it down and the one who witnesses it". He said: "All of them are equal in sin." [Muslim], An-Nawawi said: "This is evidence that it is forbidden to help in any evil."
Allaah Knows best.