you can make use of the Odoo cr (database cursor) object and execute raw SQL queries. Here's an example of how you can update a single field and avoid the rollback caused by raising a UserError:

from odoo.exceptions import UserError

# ...

def your_confirm_method(self):
# Get the current record's ID
record_id = self.id

# Get the database cursor
cr = self.env.cr

# Update the field directly using a raw SQL query
query = """
UPDATE account_move
SET your_flag_field = true
WHERE id = %s
"""
cr.execute(query, (record_id,))

# Check your condition and raise UserError if needed
if your_condition:
raise UserError("Your warning message")

# Continue with the normal invoice confirmation process
# ...

In the above code, replace your_flag_field with the actual name of your flag field in the account_move table. Make sure to replace your_condition with your actual condition for showing the warning message.

By using the database cursor (cr) and executing a raw SQL query, you can update the specific field directly in the database without relying on the ORM's transaction handling.