Deuteronomy 23:21 (KJV)

Thou shalt not lend upon usury to thy brother; usury of money, usury of victuals, usury of any thing that is lent upon usury:

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.

When thou shalt vow a vow unto the Lord thy God, thou shalt not slack to pay it: for the Lord thy God will surely require it of thee; and it would be sin in thee.

But if thou shalt forbear to vow, it shall be no sin in thee.

That which is gone out of thy lips thou shalt keep and perform; even a freewill offering, according as thou hast vowed unto the Lord thy God, which thou hast promised with thy mouth.