It was legitimate to take a return above the sum lent from the religious stranger (Deuteronomy 23:20). A tenth of this increase would therefore be tithed to God, thereby extracting from the unregenerate at least a portion of the tithe that all men owe to God.
My two cents
While I remember what L. Ray Smith said about tithing under the New Testament, I nonetheless like North’s religious view on the matter of loans, interest, and tithing.
North, G (1973). “Stewardship, Investment, and Usury: Financing the Kingdom of God” in the Institutes of Biblical Law (Volume 1). The Presbyterian and Reformed Publishing Company. p. 800.