An IAV token can only be used with Dwolla.js. To verify accounts using only the API you have to use micro deposits.
After you create the funding source you can call it’s initiate micro deposits link. It’ll take Dwolla 1-2 days to deposit the micro deposits in that funding source.
When Dwolla’s deposited them they’ll call with a webhook topic “customer_microdeposits_completed”. If you get the funding source it’ll have a new link on it to verify the micro deposits. After successfully posting the micro deposit values the funding source will be verified and Dwolla will pull the micro deposits from the funding source. If you fail you only have 2 more tries before you have to restart the micro deposit flow.