Hi Jamal,
May I know where you customer bought the Office 365 subscription through our Syndicate partner or directly from Microsoft Official Office 365 website.
If your customer only wants to renew existing E3 license subscription, then there is no need to purchase additional E3 licenses for renewing.
However, auto-renew is turned on by default. When auto-renew is on, your customer subscription will automatically renew each year on the day he subscribed.
For your reference:
Renew Office 365 for business
Moreover, if he wants to add new users only in that situation he need to
add/buy more licenses to the subscription.
Is there any specific reason for that when your customer has purchased additional new licenses for renewing?
Could you please provide us with the detailed tenant information about your customer via the private message so that we can check and further assist you?
I have sent you private message and here is the link to access it:
https://community.office365.com/user/conversations
Best regards,
Shyamal