بسم الله الرحمن الرحيم
(Fatwa: 493/614/N=7/1438)
The banking system is based on interest. According to Islam, interest based dealing is haram and unlawful, hadith sternly warns against it. So preparing and selling softwares, specially which are used in banking system and interest based dealing is recorded therein, is helping in interest based dealing and system, hence it is not lawful for any Muslim to develop such softwares and to sell them.
عن جابر بن عبدالله رضي الله عنهما قال : لعن رسول الله صلى الله عليه وسلم آكل الربوا و مؤكله وكاتبه وشاهديه ، وقال: هم سواء (الصحيح لمسلم ، 2: 72 ، ط: المكتبة الأشرفية ديوبند) ، وقال الله تعالى : ولا تعاونوا على الإثم والعدوان (سوره المائدة ، رقم الآية : 2)
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband