What was Rava's question מאי תשיך לאו תשוך (Bava Metzia 70b)

The Rishonim (Tosafos, Ritva, etc.) ask, ר' נחמן was saying that there is an איסור דרבנן so why should we care that מן התורה you can lend money to a גוי בריבית?

There is a famous Taz in a number of places (יו"ד סי' קי"ז, ח"מ סי' ב', או"ח סי' תקפ"ח) where the Taz claims that חז"ל are not allowed to assur something that the Torah explicitly permits. He is מדייק from the לשון that is used to justify how חז"ל can make גזירות. The גמרא in Sanhedrin (46a) states שבית דין מכין ועונשים שלא מן התורה ולא לעבור על דברי תורה אלא כדי לעשות סיג לתורה. Why does the Gemara say ולא לעבור על דברי תורה? Are Chazal going to make a גזירה to tell us to violate an issur? The Taz says the pshat is that Chazal even when they make a גזירה are not allowed to be עוקר דבר מן התורה and be לעבור על דברי תורה. In other words חז"ל can not make a גזירה to prohibit something that the torah explicitly permits.

Tosafos seems to use this principle to answer our original question. Rava is asking if the Torah explicitly permits us to lend a גוי money בריבית then how can we make an issur d'rabbanan to prohibit it? It is being עוקר something מן התורה.

The Rambam writes מלוה ולוה ה: א-ב :
ומצות עשה להשיך לגוי, שנאמר "לנוכרי תשיך" (דברים כג,כא)--מפי השמועה למדו, שזו מצות עשה; וזה הוא דין תורה.

אסרו חכמים שיהיה ישראל מלווה את הגוי בריבית קצוצה, אלא בכדי חייו. גזרו שמא ילמוד ממעשיו

The Rambam seems to be against the Taz. On one hand the Rambam writes that there is a מצוה דאורייתא to lend money to a גוי בריבית, on the other hand in halacha ב he writes that there is an issur דרבנן to do so. If there is a miztva to lend money בריבית to a גוי how could חז"ל prohibit it according to the Taz?

Some acharonim answer as follows. The Taz himself writes that חז"ל are allowed to make a partial issur on something that is permitted by the Torah. In hilchos Shofar the Taz quotes a question why didn't חז"ל prohibit blowing shofar on Rosh Hashana גזירה maybe you you will fix a musical instrument? He answers with his principal, that they cannot be עוקר a mitzva. However, they were allowed to make a גזירה prohibiting shofar on Shabbos because they were not completely עוקר the mitzva.

Based on this we can answer the Rambam as follows. Since there is a heter to lend a גוי בריבית, namely כדי חייו, they were not עוקר the mitzva completely and therefore they were permitted to do it.

Tosafos writes that nowadays we are allowed to lend money to a גוי בריבית because all our business is with גויים and therefore אין לאסור ריבית שמא ילמוד ממעשיו יותר משאר משא ומתן. This is quoted להלכה in שו"ע. The poskim seem to assume that the reason for the גזירה was שמא ילמוד ממעשיו and this no longer applies today. The obvious question is what happened to the idea that גזירות are לא פלוג and even if the reason no longer applies the gezera still applies?

R' Shachter in his sefer ארץ הצבי says the following. We only say that a גזירה is לא פלוג when the reason for the גזירה is not written into the גזירה, which is most גזירות. However, based on the Taz, in a case where the גזירה is coming to assur something that is explicitly permitted by the Torah, חז"ל were not allowed to make a לא פלוג but rather had to incorporate the reason for the גזירה into the גזירה. Therefore, the גזירה is limited to when the reason applies. By doing this, they are not being עוקר דבר מן התורה because the גזירה only applies in limited circumstances. This is why רבינא says that it is permitted for a תלמיד חכם to lend money בריבית. The whole reason for the גזירה is שמא ילמוד ממעשיו that doesn't apply to a תלמיד חכם. Again we can ask, what about לא פלוג? Generally all גזירות apply to תלמידי חכמים as well. The answer is that ריבית is not a regular גזירה because it is coming to be עוקר דבר מן התורה, rather it is limited to those cases where the reason applies. Based on this, we understand the heter of Tosafos. Since the reason doesn't apply in this specific case the גזירה doesn't apply, rather the גזירה is only limited to cases that fit the reason. Therefore nowadays where the reason doesn't apply it is permitted to lend money to a גוי בריבית.