You're correct. At this time there is no way to un-fulfil an order, as fulfilment is generally finalized. You'll need to keep track of the order offline to ensure that it can be fulfilled in a timely manner. It may be worth reaching out to your customer to clarify that the order has not yet been fulfilled.
Talk to you soon!
I will launch a feature request with our developers to see if this functionality can be added in the future. While I can't guarantee that this feature will be added, we're more than happy to pass along suggestions some shop owners may have that would provide greater versatility to the platform.