Hi @jmtichell – on a high level, once a masspayment is created in the Dwolla API, a job gets created in the queue which is picked up by a service that creates individual items off of the masspayment. The job starts running almost immediately, depending on the queue and the numbers of items in a masspayment. Essentially, there isn’t a way to cancel a masspayment as a whole once it get’s created, unless it’s created as a deferred masspayment like you mentioned.
As another alternative to cancelling individual transfers created from the masspayment, you could also listen to the webhook that gets sent for the creation of the debit portion of the masspayment and cancel it, which subsequently cancels the individual credit portions of the transfers. This discuss post by our team member is also a good read on masspayments in Dwolla - Mass Payments - Pay 5,000 People in Five Simple Steps
Hope that helps! Please let me know if you have any questions!