Cashier - Problem with changing Subscription quantities

This is more an issue with Stripe but don't know a solution to get around this issue:

For example, a Customer signs up with 10 users @ $10/mo and is charged $100 immediately.

Then half way through the month, the Customer reduces the number of users to 5; this is fine, but I don't want to refund/credit Customer anything back. I realize I can use the noProrate() function for this.

Then a week later, then Customer decides to increase the number of users to 15; now since they already paid for 10 users for the month, I only want to charge them the prorated amount for the additional 5 users for the remaining of the month.

If I use the noProrate to decrease the number of users, then when I increase to 15 users, Stripe will charge the customer for 10 users prorated the rest of the month.

If I prorate the decrease and prorate the increase; then the customer is charge a higher amount than just for the 5 additional users.

The goal here is when a customer signs up for a subscription of 10 user (for example); he can change the quantity as much as he wants during the subscription as long as he doesn't increase it above what he started the subscription with. If the customer increases the quantity above the original 10 during the active subscription; then I only want to charge the customer the number of users over 10 and prorate them for the remainder of the subscription.