What is "zinskauf"

Zinskauf was a financial instrument, similar to an annuity, that rose to prominence in the Middle Ages. The decline of the Byzantine Empire led to a growth of capital in Europe, so the Catholic Church tolerated zinskauf as a way to get around 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 Sermon on the Mount, and criticizing the Catholic Church for violating usury laws in spirit if not in the letter of the law.