Releasing marketplace modules from a mapped domain
Jamroom Developers
How it works is if you go to release an item from a domain, the system will call that module's URL on the domain to see if it is still installed - if it is you will get that notice. So you cannot "release" the module from a domain you are still using it on.
So you need to delete the module from "firstdomain.com" THEN release it.
You say "Clearly, I can't go to firstdomain.com to delete it" - why not? If the domain is no longer in your control you will need to contact the new owner and have them remove the module.
Let me know if that helps.