"His view of property is thoroughly mediaeval. It is identical with that of the scholastic doctors. Nummus non paret nummum (Money does not produce money), was for him, as for them, a fixed principle. Any effort to make money productive seemed to him to be sinful, contrary to the law of nature, and a violation of the laws of God, contained in the Old and the New Testaments. It had its roots in avarice, and the fruit of avarice is usury. That many of the practices which he rebuked are fundamentally dishonest, is a fact that no one will deny; but it is also a fact that Luther had no more idea of economic laws, as we understand them, than he had of the law of gravitation.
In estimating his views, we have also to take account of his own personal attitude toward wealth. Few men have ever lived who were more utterly indifferent to money. For him it was not a thing to be striven after, but only a means of livelihood and a resource with which to relieve the necessities of others. For this reason he was sure to see avarice where others might see only prudence."- editors comment
Source: Works of Martin Luther, Volume IV (Philadelphia: The Muhlenberg Press, 1931),10.