Thanks for writing in!
To resolve your issue, simply go to X/Pro > Overview and find the Revoke Validation link.
Once revoke, the other person can now insert his own license provided that the license is already assign to the domain that he is currently validating. For further details how to properly validate the theme, please check it here:
As your license has been revoke already, you can go to your licenses page (https://theme.co/apex/licenses) and re assign it so that you can use it in another domain.
Hope this helps.