What I want to do is to get the verification transaction associated to an update on the a billing info as soon as it happens: As far I can see when such event occurs, the recurly back-end raise only a "billing_info_updated" webhooks without giving any clues about the transaction associated to this event.
It is weird, because in case of failed verification on the billing info, instead, a failed_payments webhook with that transaction is raised properly.
Can you give me any isights on how to fetch this transaction in case of a successful verification?
When you update billing information on an account, the API/Recurly.js response will immediately return a 200 success, or an error (https://docs.recurly.com/api/billing-info#update-billing-info-credit-card).
If the 200 is returned, you can then immediately query the account for the last transaction, which should be that authorization transaction: https://docs.recurly.com/api/transactions#lookup-transaction
If an error is returned, the billing information is not updated, which is why only the failed_payments webhook is sent.