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.
If you know that this programme which you design will be given to a bank which deals with Riba (interest or usury) and will be used in bank transactions, whether this is about calculating interests, or savings or loans, then working on such a programme is not permissible as it is helping the bank which deals with Riba in its forbidden work. The work of a bank which deals with Riba is a complete system which ends up with Riba which is forbidden by Allaah. Hence, helping the bank with such a programme is helping in sin.
Allaah Says (what means): {…but do not help one another in sin and transgression.}[Quran 5:2]
Therefore, you should stop working on this programme which is designed for a bank that deals with Riba, and work in other permissible fields of the company where you work.
For more benefit, please refer to Fatwa 83019.
Allaah Knows best.