Responsa su Deuteronomio 7:2

It seems even more preferable to explain, based on the law of the holy Torah, that the power of an oath is great, and no distinction should be made between swearing to a Jews and swearing to a gentile. To wit, God commanded us regarding the seven nations: “you must doom them to destruction: make no treaty with them and give them no quarter” (Devarim 7:2). It is further stated: “In the towns of these peoples, however, which the Lord your God is giving you as a heritage, you shall not let a soul remain alive. You must doom to destruction the Hittites and the Amorites, the Canaanites and the Perizzites, the Hivites and the Jebusites—as the Lord your God has commanded you” (ibid. 20:17). Nevertheless, when the two spies swore to spare Rahab and her family and all her possessions, the Jews indeed upheld and carried out that oath. They utterly destroyed everything in the city of Jericho, from man to animal, by sword, but Joshua commanded to spare Rahab and her entire family from death and to keep them alive among the Israelites, as described in Ch. 2 and Ch. 6 of the Book of Joshua.