If the iPad was unsupervised you will not be able to release the code.
http://support.apple.com/kb/HT5188
Removing apps from devices
For supervised devices, if you use Apple Configurator to remove an app from a device, Apple Configurator marks the corresponding redemption code as available. It can be reassigned to a different device supervised by this copy of Apple Configurator for this Mac only. It is not available for redemption by other Macs or iOS devices.
If you unsupervise a supervised device, all apps will be removed from the device and the redemption codes used to install those apps will be marked available for reassignment. If you simply remove a device record from Apple Configurator, the redemption codes for any apps on that device will not be available for reassignment.
For unsupervised devices, there is no way to remove the app in a way that allows you to reuse or reassign the redemption code. When a code is used to install an app on an unsupervised device, it is permanently consumed.