KhadashPay
The KhadashPay project is an open-source payment system.
... (STM32F401CCU6 Version): https://www.instructables.com/KhadashPay-V30-STM32F401CCU6-Version/
V3.0 (ESP32 Version): https://www.instructables.com/KhadashPay-V30/
V3.5 (ESP32 Version): https://www.instructables.com/KhadashPay-V35/
The version of the KhadashPay that stores the data in the cloud and allows you to operate several payment terminals is available on https://sourceforge.net/projects/khadashpay-firebase-edition/