I’d like my newly hired freelancer to have access to purchase as many photodune images as he’d like, but I’d like to not give him full access to my account. Is there a way to setup sub-accounts that would allow him to use my credits to purchase images?
The only thing you can do is to download items and send it via https://wetransfer.com or similar service with expiration links.
You must trust your freelancer 
Cheers!
Try asking your freelancer to send you the images link and then you can purchase it.