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.
The transaction in reference is not clearly described in the question. There are two possible scenarios. The first is that the manufacturer sells the car to the bank for 70,000 and after the bank buys it and takes it into its possession, it sells it to the client for 80,000. This is a muraabahah transaction and there is no religious impediment to partaking in it as long as no fines are imposed on delayed payments. Please, refer to fatwa 216769.
The second is that the bank pays the 70,000 to the manufacturer on behalf of the client or gives him the money in cash form (to buy the car himself) and then takes it back as 80,000. This is interest. The fact that the car manufacturer is giving the same discount to the bank that he gives to the client is irrelevant unless the bank buys the car with a valid and actual sale transaction from the manufacturer and then sells it to the client. However, if the bank merely pays the price of the car to the manufacturer on behalf of the client, then this is considered a loan. What counts is the amount paid on his behalf, and whether a discount is granted or not, it is for the client and not the bank. That is because the bank is only an agent for the client in making payments.
Prohibited tricks do not make dealing in interest lawful; on the contrary, it increases the gravity of the sin as it resembles the actions of the Jews who used to employ tricks in violating the prohibitions of Allaah. The Prophet said, "Do not commit what the Jews have committed and violate the prohibitions of Allaah using the meanest of tricks." [Ibn Battah] It was narrated on the authority of Ayyoob As-Sakhtiyaani that he said, "They try to trick Allaah as if they are tricking young boys; were they to commit the sin bluntly, it would have been less grave..." [ʻAwn Al-Maʻbood ʻala Sunan Abu Daawood]
Allaah knows best.