Paid iPad version for iOS application

Paid iPad version for iOS application

I've been doing some research, but I couldn't find any useful information
for this question.
This is the thing:
I have a free app published on the iOS AppStore for seeing grades directly
from my University platform. I'm planning to enable in-app purchases for
downloading files from the platform and, also, for the iPad version (it's
only working on iPod Touch/iPhone for the moment).
Well, as I said, I want to charge a fee for enabling the iPad version. I
tried some things, including modifying the AppDelegate, with no success.
When I enable the MainStoryboard for iPad, iOS automatically takes it
instead of the iPhone one. I'd like the app to work with the iPhone
version of the Storyboard until the user pays for the iPad one.
Is this possible? How would you do it? Thank you.