There is a way to trigger bank transfer processing the Sandbox via the API. This is what the Sandbox Dashboard’s “Process bank transfers” button uses to simulate bank transfer processing (As mentioned in the post below). You can also reference in that thread how to trigger an ACH failure for a transfer going to or from a specific bank attached to a Customer.
To simulate bank transfer processing, you’ll call the following endpoint which will process or fail the last 500 bank transfers that occurred on your Sandbox account or the API Customer accounts you manage.
POST https://api-uat.dwolla.com/sandbox-simulations
Accept: application/vnd.dwolla.v1.hal+json
Content-Type: application/json
Authorization: Bearer {Your access token}