Is charging interest moral? Is it economically sound?
The Torah prohibits you as a Jew from charging a fellow Jew interest on a loan. However, if you do, the money is yours to keep. The point is that if you decide to return the interest, not only are you not obligated to do so, you cannot. You can “gift” it, but not “return” it.
The obvious question is, why? Isn’t loaning money with interest good for business? (We are not talking about loan sharks.) With an interest-bearing loan, both parties agree to the terms – happily. So why does the Torah prohibit such an interaction?
Charging interest makes business sense. Since there is nothing wrong with doing so purely from a business perspective, we can extrapolate that the money actually belongs to the lender.
However, from a spiritual perspective, everything that belongs to us, all that we accumulate, has come into our possession for a reason. Nothing happens by mistake and we have to be a good steward with our possessions.
So although we are interested in making money with our money, there is a law that tells us not to charge interest. This law comes directly from G-d – which isn’t to say that the money does not belong to us, because it does - but for another reason: G-d wants us to be good stewards of the money entrusted to us. Are we willing to help those who are less fortunate than us? As a favor, and not as a business transaction? If we turn a loan into a business transaction, that is our choice, and the profit is ours. But the opportunity to help our fellow Jew was missed.