How to know the refunded amount for the remaining time on a subscription upgrade?

Hi, I am implementing subscriptions levels and I have one doubt that is, when a user upgrades its subscription, it will get refunded for the remaining time of the actual subscription and after this will be charged for the upgraded subscription. What I need to know is how much was the amount refunded for that remaining time for the previous subscription, I know I get a CANCEL notification for this first, before getting the INTERACTIVE RENEWAL for the newly upgraded subscription, but with the CANCEL notification I can only get the cancellation_date but not the amount.

Same doubt when I get a CANCEL notification maybe because some user cancelled through Apple Customer Service a subscription with me, how can I know the acutal amount refunded.

Thanks very much!