How do these merchant fees work? And is this something possible via Stripe?
You can use Stripe Issuing (https://stripe.com/issuing) to issue virtual or physical cards. If you have a large transaction volume or a unique business model, you can Contact Sales from that page to discuss "interchange revenue sharing", which seems to be what you're describing.
If you want to learn more about Interchange, check out this guide: https://stripe.com/guides/introduction-to-online-payments#co...