Answer: One quote (not from the Bible) comes immediately to mind concerning loans, charging interest and other financial dealings (especially with friends). It is that before we lend money to a friend we need to decide which of the two we need the most, because we are sure to end up with one or the other - but not both! This quote certainly rings true. Many good friendships have been destroyed by lending just a few dollars that were never repaid.
God, thousands of years ago, established rules for making loans and charging interest. He told ancient Israel that at the end of every seventh year they were to cancel ALL debts of those who owed them any money.
In other words, at the end of every seventh year (beginning when the tribes of Israel entered the land promised by God) all the Israelites who lent to their fellow Israelites were to consider the debt owed them as paid in full. On the other hand, if any loans were to anyone outside the tribes of Israel, the creditors did not have to release the debt.
At the end of every seven years you shall make a release. And this is the manner of the release: Every man who has a loan to his neighbor shall release it. He shall not exact it from his neighbor or from his brother because it is called the Lord's release. You may exact it from a foreigner, but your hand shall release that which is yours with your brother (Deuteronomy 15:1 - 3, HBFV throughout).
If a fellow Israelite wanted to borrow money during the seventh year, the creditor was not to reject the request by saying, "Hey, it's the year of release. I could lose everything, so come back next year" (Deuteronomy 15:9 - 10 paraphrased).
Old Testament teaching
In modern times, it is not entirely certain what year would be the year of release. We are, however, to provide loans to our brothers and sisters in the faith. Additionally, not only should Christians offer credit to other believers when they are in the need, they are commanded not to charge a single penny in interest. If money is loaned to someone who is not a Christian, however, a fee may be charged.
If you lend money to one of My people who is poor among you, you shall not be to him as a money-lender, neither shall you lay upon him interest (Exodus 22:25).
You shall not lend for interest to your brother, interest of silver . . . You may lend on interest to a stranger, but you shall not lend on interest to your brother . . . (Deuteronomy 23:19).
A New Testament principle
What does the New Testament teach regarding loans? Jesus tells us to give to him that asks (e.g. requests a loan), and not to turn him or her away empty-handed.
And if anyone shall sue you before the law and take your garment, give him your coat also. And if anyone shall compel you to go one mile, go with him two. Give to the one who asks of you; and do not turn away from the one who wishes to borrow from you (Matthew 5:40 - 42).
Jesus, in the book of Luke, makes it clear that we are not even to turn our enemies away in their need (Luke 6:30, 35), even if our loans or any interest it may generate is never paid back. Other Bible verses to study concerning lending include Matthew 25:35 - 40, 2Corinthians 9:6 - 7 and James 1:27, 2:15 - 16.