الحمد لله رب العالمين ، وصلى الله على سيدنا محمد وعلى آله وصحبه أجمعين، وبالله التوفيق

فقد أجمع المسلمون على تحريم الربا وعلى أنه من الكبائر ,المجموع: 9: 295

Imam An-Nawawi said: “The Muslims have consensus that Riba (usury) is Haram, and that it is from the major sins.” (Majmu’ 9:259)

There are two types of banking systems today, Islamic and Conventional. Islamic banking is a financial Institution that conforms to the Shariah (Islamic Law). Therefore, it is permissible to develop software for the Islamic banks. Conventional banking is a financial institution whose activities mostly are involved in interest based transactions.

The Qur’an and Hadith have severely warned people regarding riba (interest).

“Those who eat riba (interest) will not stand (on the Day of Resurrection) except like the standing of a person beaten by Satan leading him to insanity…”(Qur’an 2:275).

Sayyiduna ‘Abdullah ibn Salam (radiya Allahu ‘anhu) reports that Rasulullah said:

“One dirham of riba acquired by a person is worse in the sight of Allah I than him fornicating thirty-three times.” [Tabarani – al-Targhib wa al-Tarhib: 3: 6]

Sayyiduna Abdullah ibn Masʿūd (radiya Allahu ‘anhu)  narrates that Rasulullah  cursed the one who accepted riba, the one who paid it, the witness to it, and the one who recorded it. (Sunan Abu Dawud: Kitab al-buyūʿ: 4: 3331).

The principle governing the issue in your question is:

وَتَعَاوَنُوا عَلَى الْبِرِّ وَالتَّقْوَى وَلا تَعَاوَنُوا عَلَى الْأِثْمِ وَالْعُدْوَانِ وَاتَّقُوا اللَّهَ إِنَّ اللَّهَ شَدِيدُ الْعِقَابِ

“And assist one another in righteousness and piety, but do not assist one another in sin and transgression.” (al-Māidah: 2)

If the software is developed to calculate and charge riba (interest) such that it has no other purpose, in that case it is impermissible. If the software has several functions, and it may be used for the riba-based transactions such would be permissible, but disliked if a person doesn’t know that the bank will use it for a riba-based transaction. It would be impermissible if he knows for certain that the bank will use it for a riba-based transaction. If the software is not meant for the riba-based transaction, then this would be permissible.

والله تعالى أعلم
Allah Most High knows best.