How to disable prorating / handle this situation

I'm using Laravel Spark for my application's billing, and the users will get a number of API credits based on their selected subscription. The issue I'm dealing with is that they for example pay for a 100$ subscription, use all of its API credits, and then they downgrade to a less expensive subscription, wait for the next billing cycle, and get the API credits relevant to their current subscription, but because of the prorating, they get a say 80$ to their Customer Balance (in other words parts/months free for their less expensive subscription) from that prorated more expensive subscription, even though they used all the credits.

What am I missing here?