ACH Transfer Timing and Cancellation Windows
Overview
If a Transfer was created in error, you can cancel before it is batched and sent to the Federal Reserve. You can cancel a transfer via the cancel transfer endpoint while it is in pending status and has not yet been batched. ACH transactions batch for distribution at the top of every hour. As a best practice, submit your cancellation 10 minutes before the top of the hour to allow sufficient time for processing. After the transfer is included in that hourly batch, it can no longer be canceled.
If the POST /v1/transfers/{id}/cancel API response states that the transfer cannot be cancelled, then it indeed cannot be cancelled by any means.
When You Can Cancel
Transfers can be cancelled via the API cancel transfer endpoint. Cancel a transfer 10 minutes before the top of the hour or before its next batching. Once a transfer has been batched and released, it can no longer be canceled.
Example A: A transfer created at 9:30 AM ET should be canceled by 9:50 AM ET, ahead of the 10:00 AM hourly batch.
Example B: A transfer created at 8:30 AM ET should be canceled by 8:50 AM ET, ahead of the 9:00 AM hourly batch, even though its Next Day transmission window doesn't occur until 10:30 AM ET.
See more on ACH windows and cutoff times here: ACH Window & Cutoffs.
Settlement Schedule
The Federal Reserve's ACH Network processes payments 23¼ hours every business day and settles four times a day. Settlement occurs when the settlement service is open. The settlement system is closed on federal holidays and weekends, and on business days from 6:30 PM to 7:30 AM ET.