Are financial transaction programs that record checks and payments permissible or forbidden? And does participating in their development cause sin?

There is no sin upon a Muslim for working on financial transaction software programs that can be used for both lawful and unlawful purposes, as long as the user's intent is unknown. However, programs that are known in advance to be used exclusively for usurious transactions are impermissible to work on, due to the assistance it provides in sin and transgression.