Unfortunately, I think you're unlikely to find anything besides the ones made by the big companies: Google, Apple, Samsung, and Garmin are the ones I know. They each have agreements with the banks and credit card companies to handle the secure exchange of data required for the touch payment system. In fact, there are still some issues resulting from a lack of cooperation (such as Citi Bank not working with Garmin Pay because they can't be bothered to set up the relationship). I imagine an open source software would be unable to get the banks to pay attention to them to establish a partnership, or would otherwise be declined because the financial institutions wouldn't trust them.

I have looked around, myself, but not found a FOSS alternative. There are typically compliance issues like PCI DSS for certain banks that prevent trust and cooperation from those banks outside a larger entity like Google, Apple, etc.

Aside smart phones, Flipper Zero can clone some cards sucessfully. But that's an entirely different device, not an app for a phone. Best of luck!

Unfortunately AFAIK there are no open source projects that major credit card vendors and banks are willing to work with. That's the fault of the banks and cc vendors, not the vendor you are shopping at of course.

You might be interested in strike. It's not open-source but it's built on open-source protocols:

• Send and receive USD and BTC to more than 60 different countries, seamlessly convert between the two. Low or no fees depending on what you're sending/receiving and where. Vastly cheaper than western union and other remittance services.
• Transactions settle in seconds thanks to lightning network (there's your open-source protocol)
• Ability to scan QR codes to make or receive payments, or create QR codes to request them. Basically works as a point-of-sale.
• You can top-up or drain your account much like venmo (link to debit card or bank account) and you get your own unique username which you can change whenever you want.

Electrum

A shame that this search turned out null, but at least I learned about https://www.opensourcealternative.to/

I use iCard. It's not open source but it's not Google or Apple, which is the most important.

Are you in Europe? Then OpenBank