Paraphrase "Superfluous wealth can buy superfluities only. Money is not required to buy one necessary of the soul."
What Thoreau is saying here overall is essentially that money cannot buy happiness. It cannot buy what you really need in order to be a fulfilled human being.
The first sentence can be paraphrased something like this:
The only thing that you can buy with money you don't need is things that you don't need.
The second sentence can be paraphrased something like this:
The one thing that your soul really needs is something that cannot be bought with money.
So overall he is saying that we should not try hard to get a lot of money. This was a major idea of transcendentalism -- if we chase money too much, we will never become truly happy.