The redemption code model is deprecated by Apple. We recommend distributing licenses through Volume Purchase Program when possible.
To deploy a paid iOS app to your managed devices via Systems Manager, you must obtain the requisite number of redemption codes through Apple's Volume Purchase Program (VPP). Through VPP, you will obtain a unique redemption code for each app you purchase for deployment (i.e., 20 codes for 20 copies of the app). Simply copy/paste the redemption codes you receive from Apple into the text box that appears on the MDM > Apps page when you add a paid app.
Once the app is deployed through Systems Manager to your specified iOS device, the Apple ID that is used to download/install the app on the iOS device will permanently bind to the VPP redemption code. The first device to redeem a particular code will then link to the redeemed code in the app text box on the MDM > Apps page. The same redemption code can never be used by another Apple ID, and the same Apple ID can never use multiple codes for the same paid app. You may, however, downloaded the paid app to any device using the same Apple ID that originally claimed the code (subject to Apple's user licensing agreement).
Note: It is not advisable, once a paid app is deployed, to manually remove and re-add a code from the redemption code box, or delete and re-add a paid app altogether. When re-added, a code will be considered new/available by Systems Manager despite the fact that the code is, in actuality, used/redeemed.
For more information on Apple's Volume Purchase Program for Education and Business, see: http://www.apple.com/education/volume-purchase-program/ and http://www.apple.com/business/vpp/