What is the ruling on designing and developing an electronic payment system using MasterCard and Visa cards that connects to conventional banks to verify the soundness of the payment process and transfer the amount to the seller's account, knowing that there are no Islamic banks in Tunisia?

If the prepared system is used in both permissible (halal) and impermissible (haram) transactions, then there is no sin upon you for preparing it. However, it is more prudent to avoid it if it is highly probable that it will be used in interest-based (riba) transactions. But if it is used only in interest-based transactions, then it is not permissible for you to prepare it.