Zinskauf (pronounced as /de/, "purchase interest") was a financial instrument, similar to an annuity, that rose to prominence in the Middle Ages.[1] [2] [3] The decline of the Byzantine Empire led to a growth of capital in Europe, so the Catholic Church tolerated zinskauf as a way to avoid prohibitions on usury. Since zinskauf was an exchange of a fixed amount of money for annual income it was considered a sale rather than a loan. Martin Luther made zinskauf a subject of his Treatise on Usury[4] and his Sermon on Trade and Usury[5] and criticized clerics of the Catholic Church for violating the spirit if not the letter of usury laws.
In one historian's analysis: