Is there a law anywhere in black and white on paper that says it is against the law to keep a possum that was rescued as pet since if would only die if released into the wild?
There sort of is a law, this one. It does not absolutely forbid owning a possum, but it is required that you have a license to take or keep a wild fur-bearing animal including a possum. There is no existing "license to take and possess as a pet", but you could try under trapping licenses. However, they warn that the
Fur-bearing Propagation Permit does not authorize individuals to possess live fur-bearing animals as pets.This IS NOT a “pet permit.”
Ultimately, what would happen is you apply for a license, but they would deny the application because you don't plan to propagate the animal, or skin it. Although the statute does not specifically prohibit keeping a fur-bearing animal as a pet, it does authorize Parks and Wildlife to promulgate and enforce regulations.
Another approach would be to seek a wild animal rehabilitation permit, pursuant to PWD regulations (here). This is also not a general pet permit, but in case your program of possum rehabilitation fails, you are not required to kill it or release it into the wild to die. There is a test required, to be sure you know how to rehabilitate wild animals, plus a requirement for letters of support from conservationists, veterinarians or rehabilitators.