Nope, that is not really an option. I have not found a native faction that would sell lands.

With the little poisoned pill they have just set him up.

If he attacks the Cherokees then he has broken a treaty and will have everyone on his back, even though there was no treaty...they just gave military access so he would take the hit.

Nothing is to be done if you want Louisiana but to take them out. It is just a set up and you have to live with it.