Rabbeinu Bahya, Devarim 23:21

לנכרי תשיך. אסר הרבית מישראל גם בעשה. ודעת הרמב"ם ז"ל שזו מצות עשה, וחשבה מכלל רמ"ח מצות עשה. ויש שפירש לנכרי תשיך שאינה מצוה אלא רשות, כמו (שמות כ׳:ט׳) ששת ימים תעבוד, וכונתם שהתורה לא קבעה מצוה בזה על כל פנים להלוות לעכו"ם ברבית אלא שהרשות בידו אם ירצה אם לא, כי לא אסרה אותו. ומה שאמרו במסכת מכות (תהילים ט״ו:ה׳) כספו לא נתן בנשך, אפילו רבית דעכו"ם, אין להבין מזה אסור אלא דרך גדר וחומרא הוא במי שנוהג באותן המדות והוא גודר עצמו בגדרים וסייגים שיזכה לאותן המעלות הנזכרות באותו מזמור, וכבר התירו רז"ל בפירוש רבית העכו"ם בכדי חייו, והוא שאמרו בבבא מציעא אסור להלוות ברבית לגוי ביותר מכדי חייו אא"כ הוא תלמיד חכם, גזרה שמא ילמד ממעשיו. ומה שאמרו בספרי לנכרי תשיך זו מצוה עשה, לא לענין שהתורה קבעה בזה מצות עשה, אלא כנגד המלוה ברבית לישראל חברו אמרו כן, שהמלוה לישראל ברבית עובר על מצות עשה דלנכרי תשיך, שכן משמע לנכרי תשיך ולא לישראל, ולאו הבא מכלל עשה עשה. וכיוצא בעשה זו דרשו בספרי בענין דגים (ויקרא יד) את זה תאכלו, זו מצוה עשה. וכל אשר אין לו, זו מצות לא תעשה. וכי חייב הוא לאכול דגים טהורים, אלא הכונה היא שאם אכל דגים טמאים עבר על עשה דאת זה תאכלו ועל לא תעשה של וכל אשר אין לו, וכן בכאן ברבית בא ללמד על המלוה לישראל ברבית שעובר בעשה ולא תעשה דלנכרי תשיך, כלומר ולא לאחיך, ולאו הבא מכלל עשה עשה, ולא תעשה דולאחיך לא תשיך. ודרשו רז"ל, ולאחיך לא תשיך שעושה מעשה אחיך, וזה לרבות את הגרים שאסור לך להלוותן ברבית, ולהוציא זרעו של עשו שאעפ"י שכתוב בו (במדבר כ׳:י״ד) כה אמר אחיך ישראל, אחרי כן הותר, שכן הזכיר הנביא (עובדיה יא) ביום עמדך מנגד ביום שבות זרים חילו ונכרים באו שעריו ועל ירושלים ידו גורל גם אתה כאחד מהם, למדך שכבר נתבטלה האחוה ויצאה מכללה והרי הוא כשאר האומות הזרים, שהרי הרבית לא נאסר לישראל אלא מטעם שנצטוינו להחיותו ולעשות עמו גמילות חסדים, ממה שכתוב (ויקרא כ״ה:ל״ו) וחי אחיך עמך. לנכרי תשיך, “to the Gentile you may (or must) charge interest.” The prohibition of charging a Jewish borrower interest is at the same time a positive commandment to charge a Gentile interest on loans extended to him. Maimonides, in Hilchot malveh veloveh 5,1 rules that our verse is definitely a positive commandment, not merely permission to accept interest from a Gentile borrower. Others, such as Ibn Ezra, interpret our verse as merely granting permission to charge interest to Gentile borrowers. This statement is supposed to be similar to that of ששת ימים תעבוד, “during six days you are to work” (Exodus 20,9) which is a permission. The Torah permitted loans to Gentiles, and it permitted charging interest on those loans.
When the Talmud in Makkot 24 states that the words בספו לא נתן לנשך, “he has never lent money at interest” (Psalms 15,5) includes that the person referred to has not charged interest to a Gentile, this is not to be understood as a prohibition but is a voluntary restriction imposed upon himself by the lender. A person practicing such virtues will attain the spiritual level described as desirable in that psalm.
A statement by our sages in Baba Metzia 71 that it is forbidden to lend to a Gentile against excessive interest except if the lender was a Torah scholar, has to be understood against the following background. [The whole passage is strange, compare Rashi and Tossaphot. It is assumed that the Gentile had called the Jew “wicked,” had insulted him. Ed.]. Ordinary Jews may not charge an amount of interest which would reduce the income (net) of such a Gentile by more than a third. The restriction is designed to ensure that Jewish lenders do not learn bad habits from the Gentiles. Torah scholars are not presumed to copy such bad habits. When the Sifri writes that the words לנכרי תשיך are a positive commandment, the meaning is not that the Torah imposes an obligation on an Israelite to grant loans to, and to charge interest to Gentiles, but it addresses itself (obliquely) to the Israelite charging interest on a loan to a fellow Jew as also being guilty of violating a positive commandment, (not merely a negative commandment). The verse contrasts conduct vis-a-vis a Gentile with that towards an Israelite. The words “to a Gentile you must charge interest,” are a restatement of the prohibition not to do so to a fellow Israelite. It is something called לאו הבא מכלל עשה, “a negative commandment implied in a positive commandment.”
In a similar fashion we find that Sifri Re'ay 103 understands the commandment “these you may eat” dealing with permissible fish (Deut. 14,9) as meaning that all the fish not enumerated in that verse as possessing certain identifying marks such as fins and scales are forbidden for Jews to eat. The line: “these you may eat,” certainly does not mean: “these you must eat.” The Torah simply says that if we eat fish which do not have fins and scales we have violated a positive commandment called “these you may eat.” At the same time one has violated the negative commandment “these you must not eat,” which is appended to “all those which do not have, etc.” The same holds true for the commandment dealing with interest in our Parshah. Our sages interpret the words ולאחיך לו תשיך, “and to your brother you must not charge interest,“ as referring to people acting like your brother, i.e. including recent converts, people who have now become like your brother seeing they are bound by the same rules of conduct as your brothers. At the same time this phraseology excludes the descendants of Esau as, though they are biologically related, they do not adopt the rules of conduct of your brethren. Hence one may or must charge interest on loans made to Edomites. Although in Numbers 20,14 Moses had described the Jewish people as “your brothers,” when speaking to the Edomites, the legal status of these people changed after they demonstrated that they did not consider themselves as “brothers.” It became perfectly permissible to charge interest on loans to Edomites. There is a statement by the prophet Ovadiah 11 ”on that day when you stood aloof, when aliens carried off his goods, when foreigners entered his gates and cast lots for Jerusalem, you were as one of them.” This underscores the point we just made.
The whole reason for the prohibition of charging interest is based on our obligation to keep fellow Jews alive, to relate to them with deeds of loving kindness seeing the Torah wrote וחי אחיך עמך, “ensure that your brother will be able to live alongside you” (Leviticus 25,36). No such directive exists concerning Gentiles. A Jew who has decided to practice idolatry is also not subject to the law not to charge interest on loans to him. Seeing that the Torah made it clear that such a person’s very body is unprotected by Biblical injunctions, his property is most certainly not protected by Torah law! We find confirmation of this in Avodah Zarah 26: “one may push drowning idolaters (Jews) down into the pit and one is not allowed to pull them up (in order to save their lives).” On the other hand, accepting loans from such people and paying interest on such loans is probably forbidden as opposed to natural born pagans. The reason is that the Jewish heretic was born with the obligation to conduct himself as Jew, whereas the same cannot be said of the natural born pagan. Our sages in Sifri Ha'azinu 308 interpret the word (Deut. 32,5) בניו מומם as meaning: “although they are blemished they are still His children;” in other words, loans accepted from such people are considered as loans extended by Jews. At the very least, if one accepts a loan from such a Jewish idolater one violates the negative commandment ולפני עור לא תתן מכשול, “do not place obstacles in the way of a blind person. “