What are the TN laws regarding an heir of joint heir property to sale their interest/shares to another heir of the same property?

Is this before the testator has deceased? I'm not sure about specific TN law, but generally anyone can sell their ownership in a future asset.

The purchaser may take the risk that the inheritance will not be changed or may put into the contract that if the inheritance doesn't materialize, that they will be reimbursed.

But I don't see a problem with entering into a contract to sell their share.