Deuteronomy 23:20
Unto a stranger thou mayest lend upon usury; but unto thy brother thou shalt not lend upon usury: that the LORD thy God may bless thee in all that thou settest thine hand to in the land whither thou goest to possess it. Unto a stranger thou mayest lend upon usury; but unto thy brother thou shalt not lend upon usury: that the LORD thy God may bless thee in all that thou settest thine hand to in the land whither thou goest to possess it.
Interest may be charged on a loan to a stranger or foreigner, but no interest is to be charged to a brother or fellow Israelite. This means a wealthy believer may receive interest from unbelievers, yet must not take interest from a poor fellow believer. Instead of using a brother's need to increase one's own wealth and profiting from his distress, one is to relieve him in his need.
The prohibition protects a brother in need from being exploited or impoverished for another person's gain. Even the permission concerning foreigners does not allow exorbitant or oppressive terms. Lending under this rule is to be guided by mercy and justice rather than by taking advantage of another's necessity. Obedience to this rule brings the Lord's blessing on all the work one sets the hand to in the land to be possessed.