Marketplace vendors are able to generate and provide Promotion codes for customers. This allows vendors to grant specific percentage discounts on their apps. This is currently only available for Server apps, for more information on this please see:
The Atlassian Marketplace provides Vendors with Paid via Atlassian apps access to reports on the licenses and transactions that occur. Currently these reports do not contain details on when a Promotion code was applied.
There is an open feature request for this addition of this behaviour which can be tracked here:
We encourage you to become a watcher on the issue and any comments or concerns you may have relating to this.