Can I Work for a Conventional Bank?
As a principle, it should be remembered that, the permissibility or impermissibility of employment in a particular firm or company basically depends on two things:
1) The nature of the job must be lawful, meaning if the work entails something that is prohibited in Islamic Law, such as posing for nude picture-taking, serving alcohol or recording and calculating interest transactions, etc, then this type of work will be unlawful.
The reason for its prohibition is that, the work itself has been prohibited in Islamic Law. And due to this, even if the salary was paid from a lawful source, the work will still remain unlawful.
2) The salary or income that one receives must also be from a lawful source. If the work itself is lawful, but the salary is paid from unlawful and impure money, such as stolen or interest money, it will still be unlawful to accept that job.
Keeping the above two points in mind, the ruling with regards to working in a bank depends on exactly what the job consists of.
Employment at a bank that entails direct involvement with interest based transactions, such as the job of a manager, cashier, clerk, etc will be unlawful.
Sayyiduna Abd Allah ibn Mas'ud (God be pleased with him) narrates that the Messenger of God (God bless him and give him peace) cursed the one who accepted usury, the one who paid it, the witness to it, and the one who recorded it (Sunan Abu Dawud: Book 22, Number 3327).
In another Hadith, the Messenger of God (God bless him and give him peace) invoked a curse on the receiver of interest, the payer of interest, the scribe and the two witnesses. And he said: "They are the same" (i.e. in sin) (Sahih Muslim).
Thus, the warning of the Messenger of God (God bless him and give him peace) is very severe. The one who is directly involved with interest transactions in any way, shape or form, such as writing or calculating the transaction will earn the curse of God Almighty and his beloved Messenger (God bless him and give him peace).
Therefore, one must avoid accepting a job at a bank, which consists of being involved in interest transactions and dealings. It is unlawful to help and assist others in their interest-based dealings. One must decide between temporary worldly gains and the curse of God Almighty and his Messenger (God bless him and give him peace).
If a person is already employed at a bank, then he is advised by the scholars not to leave his job immediately, rather he should look for a permissible job elsewhere and when he succeeds in obtaining one, he should leave the employment at the bank.
The important point here is that one must exert all possible ways and means in order to a find an alternative job. If one puts in the effort and is willingly ready to accept comparatively reduced benefits, then there is no reason why a suitable job is not found. (See: Contemporary Fatawa by Shaykh Taqi Usmani, P. 280-281).