Rambam - Sefer HaMitzvos: Positive Mitzvah 198: Positive Mitzvah 198

Positive Mitzvah 198: Taking Interest from a Gentile
Deuteronomy 23:21 "You may take interest from a gentile"

It is universally accepted to collect interest on loans.

The Torah forbids taking this extra "bite" from a fellow Jew's pocket (see Negative Mitzvah 235).

Giving up this profit can be quite a sacrifice, as it can amount to a considerable sum!

This Positive Mitzvah teaches us that this consideration must only be shown to Jews and that we may collect interest from gentiles.

Every created being is similar to both the sun and the moon. The sun gives constantly of its warmth and its light. The moon, on the other hand, only reflects the light it receives from the sun. So too, there is nothing in G-d's world that may only take without giving. And there is nothing that may give without receiving. Every created being must both give and receive, be the sun as well as the moon.

From: Bringing Heaven Down to Earth by Tzvi Freeman - tzvif@aol.com