The Church Fathers had already taken a stand on the question of usury in the fifth century, but loans on credit were not widespread transactions during the barbarian conquest.
St. Jerome on Usury, 400: If you gave to a prosperous person you should not have done so: but if (even so) you gave it as to a needy person, why should you demand more on the score of his being prosperous? Some lenders are wont to take small gifts of dilferent value, not realizing that whatever is received over and above what was given is called usury and superabundance. They ought not to take more, however much it be, than was originally given by them.
Pope Leo the Great, 444: Neither do we think that it should be lightly passed over that some people, seized with the desire for filthy lucre, put out their money at usury in order to become rich thereby. And we have to complain of this not only with regard to those in clerical office but we likewise grieve to see that it holds true of lay people who wish to be called Christians. We decree that this should be severely punished in those found guilty, so that all occasion of sin may be washed away.
Source: From: J. P. Migne, ed. Patrologiae Cursus Completus, (Paris, 1845), Vols. XXV, p. 177, CLXXXVII, p. 959, reprinted in Roy C. Cave & Herbert H. Coulson, A Source Book for Medieval Economic History, (Milwaukee: The Bruce Publishing Co., 1936; reprint ed., New York: Biblo & Tannen, 1965), pp. 170-171.