Even Wikipedia thinks that medievals did not allow any interest, because it was thought to be usury:
Christianity
Christ drives the Usurers out of the Temple, a woodcut by
Lucas Cranach the Elder in
Passionary of Christ and Antichrist .
[33]
The first of the
scholastic Christian theologians,
Saint Anselm of Canterbury, led the shift in thought that labeled charging interest the same as theft. Previously usury had been seen as a lack of
charity.
St.
Thomas Aquinas, the leading scholastic theologian of the
Roman Catholic Church, argued charging of interest is wrong because it amounts to “double charging”, charging for both the thing and the use of the thing. Aquinas said this would be morally wrong in the same way as if one sold a bottle of wine, charged for the bottle of wine, and then charged for the person using the wine to actually drink it.
[34] Similarly, one cannot charge for a piece of cake and for the eating of the piece of cake. Yet this, said Aquinas, is what usury does. Money is a medium of exchange, and is used up when it is spent. To charge for the money and for its use (by spending) is therefore to charge for the money twice. It is also to sell time since the usurer charges, in effect, for the time that the money is in the hands of the borrower. Time, however, is not a commodity for which anyone can charge. In condemning usury Aquinas was much influenced by the recently rediscovered philosophical writings of
Aristotle and his desire to assimilate
Greek philosophy with
Christian theology. Aquinas argued that in the case of usury, as in other aspects of Christian revelation, Christian doctrine is reinforced by Aristotelian
natural law rationalism. Aristotle’s argument is that interest is unnatural, since money, as a sterile element, cannot naturally reproduce itself. Thus, usury conflicts with natural law just as it offends Christian revelation: see
Thought of Thomas Aquinas.