Debugging code that interacts with payment systems?

Question

Payment systems

Hello. The question may seem a bit broad. How to debug the code of interaction with PS?
Essence: we develop a "startup" within one company that has its own internal PS. We were provided with a description of the API, but we will not be able to get either the base url or the api key until the application is ready, that is, until the very moment of actual integration into the PS from our application. Accordingly, we will not be able to make test requests there. What can be done in such a case?
I have never interacted with even a single public PS, so can you briefly describe how at least such code is debugged? After all, we do not receive the weather on request, but do test money transactions. To do this, popular PSs have some kind of test cabinets, where we send and receive "fake" money,