“Christ himself … says in Luke 6:35, “Lend, expecting nothing in return.” That is, you should lend freely, and take your chances on getting it back or not. If it comes back, take it; if it does not, it is a gift. According to the gospel there is thus only one distinction between giving and lending, namely, a gift is not taken back, while a loan is taken back—if it is returned—but involves the risk that it may become a gift. He who lends expecting to get back something more or something better than he has loaned is nothing but an open and condemned usurer, since even those who in lending demand or expect to get back exactly what they lend, and take no chances on whether they get it back or not, are not acting in a Christian way.”
Martin Luther Selected Writings of Martin Luther 1529-1546 ed. Theodore G. Tappert (Fortress Press, 1967) 97.