Custom price for subscription

Hi, I can’t seem to find any way to set a custom price for a subscription and also setup a prorated invoice for current duration. For example, a plan costs $10 and I want to bill the user $15, but there is no way other than creating a new subscription plan and set it’s new price there. Is that even doable with subscriptions?

Also, suppose we make a subscription on 20th and want to bill the customer starting with 10 days and then on anniversary date set. How should we do that with subscriptions? Or even if that’s doable via Sales Invoice, maybe we can setup a repeating item there?