According to the supreme court why is the indian removal act unconstitutional

The Indian Removal Act is a law passed in 1830 that authorized the President of the United States to negotiate with Indian Tribes for their relocation to territories located west of the Mississippi River in exchange for the lands they were currently settled in.

However, this led to an attempt from many private entities and individuals to purchase land from Native Americans. This led the Supreme Court to adopt a stance of prohibition towards these attempts. The first case in which they ruled against an eventual purchase was in Johnson v. M'Intosh in 1823.