transaction.ach_returned webhook. See webhooks for more info.
A payment link that was marked paid because of a pending ACH moves back to unpaid or partially_paid when that ACH is returned. The same applies to each payment link on an aggregate payment link.
Testing
To test ACH returns, you can call thesandboxOnlyTriggerAchReturn on a Transaction with type: achPayment. This will simulate an Insufficient Funds return error code and send out a webhook.