## Versions:
17.0+
No fix needed in 16.0 as the field was not computed and this changes with https://github.com/odoo/odoo/commit/7e012dd5441e87ce3c2688f9280381b041b00b0d

## Issue:
A payment to be refunded is displayed as already refunded in the wizard for some users.

## Steps to reproduce:
Ensure the `Demo` payment provider is enabled in `Test Mode`. Ensure you are logged in as Admin user.
- Navigate to the `Invoicing` app:
    - Create a new customer invoice and confirm it;
- Navigate to the Portal using the invoice's `Preview` button:
    - Pay the invoice with the `Demo` provider with a `Successful` Payment status;
- Come back to edit mode:
    - At the bottom of the form, click the information icon in the totalization section;
    - Click the `View` button of the popover;
    - Click the `Refund` button and remember the data;
- Copy the URL;
- Log out and log back in as Demo user;
- Paste the URL to come back to the payment:
    - Click the `Refund` button: - See the `Refunded Amount` value equal to the `Payment Amount`; - See the `Maximum Refund Allowed` value set to 0;

## Cause:
The maximal refundable amount can't be computed for users who don't have `Payment Provider` model access rights.

## Fix:
Compute the amount in `sudo` mode to bypass user's `Payment Provider` model access rights.


opw-4396372

Forward-Port-Of: odoo/odoo#194871