Retrieving prorated refund amount and transaction ID for Apple IAP subscription upgrade

The user subscribed to a service in the app and then upgraded the subscription during the entitlement period. In this case, Apple will issue a prorated refund to the user. However, I did not receive the App Store Server Notifications V2 notificationType REFUND on the server side, nor did I find any refund records. How can I obtain the specific amount of the prorated refund and the transaction ID?

[Re-Titled by Moderator]

Original Title: When an Apple IAP subscription is upgraded, users receive a prorated refund. How can the server obtain the refund amount