I understand how you handle it on your end but let's assume I decide to build an API for a language you don't support yet. In a few months when you add new versions of the API where some fields become optional or mandatory, then won't I have to know all this as the library developer and handle each case? Because some of my users would expect to still be able to send the charge amount while others would not.

I'm more thinking in the idea of doing some "pre-validation" in my library to avoid an API call from the client when something is already expected to fail (like missing the amount field right now) which would not fail with a new version of the API. So I would have to adapt to each version to know that this user would require the amount field while another one would not.