According to this blog post, you can get the code back in Meraki if you need to-
Meraki Mobile Device Management | admin
Near the bottom it says- "
Especially if it is a paid for app you are adding make sure you tick the ‘remove with MDM’ box as this will enable you to reclaim the redemption code back at a later day of required."
I've not used Meraki so I can't comment if it is correct. I'm trying to decide myself whether it is best to go with Meraki and push apps wirelessly or Apple Configurator and connect to sync and charge trolley. From that blog post, it sounds like Meraki works nicely but I've heard a lot of people had trouble with it.