Praise be to Allaah.
We have quoted many fatwas on this site about the ruling on
working in riba-based banks, which is haraam and it is not permissible to
work in these banks. See the answers to questions no.
21113,
21166,
26771.
Your question about working in the bank is more important than the ruling on
dealing in currencies.
In addition to the fact that you are sinning by working in
the bank, you are also sinning in another way, which is deceiving people at
work and exchanging currencies at the old rate even though you know it has
changed. It is not permissible for you to take advantage of the fact that
people who are dealing with you do not know the new rate. The Prophet
(peace and blessings of Allaah be upon him) said: “He who deceives us is not
one of us.” Narrated by Muslim (151).
The believer should treat people in the way that he would
like to be treated by them, as the Prophet (peace and blessings of
Allaah be upon him) said: “None of you truly believes until he likes for his
brother what he likes for himself.” Narrated by al-Bukhaari (13) and Muslim
(45).
Undoubtedly you would think that the one who treated you in
this manner was deceiving you and tricking you.
So you must look for permissible work, and be sincere in your
dealings.
We ask Allaah to suffice us by means of that which He has
permitted so that we will have no need of that which He has forbidden.
And Allaah knows best.