When you give to a needy person or for a worthy cause, it is no more than a loan. G‑d Himself is the guarantor.
Not only that, G‑d says: The more you give, the more you get—beyond what you've given.
The Talmud tells us that this is the only case in which we are allowed to test G‑d. Donate a tenth of your earnings, and you will earn far more.


But if you get back more than you gave, and it was a "loan", then you have violated the commandment against receiving interest on a loan to your countryman (a fellow Jew). You are allowed to get back more than you gave only if it was a gift, not a loan, or it was a loan to a gentile.
And you supposedly get it back tenfold, which would violate secular usury laws as well, if it is a loan. (You are allowed to get back that much on an investment without guaranteed return, such as when you contribute to a new business in return for a percentage of any future profit that the business makes, which could be nothing, but on a loan with guaranteed return, the interest rate is typically limited to about 20-30% per year.)