Generally you can downgrade using a media & product key you/the customer already owns, for OEM licenses Microsoft will not provide any media or keys to facilitate the downgrade.
You can of course reuse a key which was used for another server already - in case the online activation does not work, you can contact activation hotline via phone and mention that you are doing a downgrade.
If the customer does not have Windows Server 2012 media or keys already, then the better option is to sell Windows Server licenses via Voume licensing since for VL customer media & keys will be provided for N-2 Versions (So currently Server 2016 and 2012 R2).
Good day @kreshnikhasani ,
You might want to address this with the Tech Community for details here: https://techcommunity.microsoft.com/t5/Windows-Server/ct-p/Windows-Server
Also this document talks about the downgrading rights.
Hope this helps,