Commentary for Deuteronomy 24:1

כי מצא בה ערות דבר [WHEN A MAN HATH TAKEN A WIFE, AND MARRIED HER, AND IT COMES TO PASS THAT SHE FINDS NO FAVOUR IN HIS EYES.] BECAUSE HE HATH FOUND SOME SCANDALOUS THING IN HER; [THEN LET HIM WRITE A BILL OF DIVORCEMENT … AND SEND HER AWAY …] — it is his duty to divorce her because she should not find favour in his eyes (cf. Gittin 90b).

כי יקח איש אשה ובעלה, When a man takes (marries) a woman and lives with her;” The Torah speaks of two methods of acquiring the woman; 1) money or equivalent. 2) marital intercourse. Whenever the Torah speaks of כי יקח, the meaning is not that someone simply “takes,” but that he acquires something with money (or equivalent). Examples of the relationship between קיחה and money are already found when Avraham wanted to purchase the cave of Machpelah (Genesis 23,13) נתתי כסף השדה קח ממני, “I have deposited the money for the field, take it from me.” There is yet a third way in which one can acquire a woman, i.e. by means of a document. The Torah alludes to this in the very next verse when a divorce is discussed. Just as a document is required to dissolve the husband-wife relationship, otherwise the woman cannot be remarried, so a marriage may be concluded by means of such a document. The words ויצאה והיתה, “she leaves and she remarries” next to one another are understood as comparing the process of leaving a marriage to that of entering a marriage (compare Kidushin 5). We therefore have three ways in which a woman can be acquired as pointed out at the beginning of Tractate Kidushin.
The exegetical tool known as גזרה שוה, “identical expressions comparing different subject matter” is so strong that it enables the sages to treat as a capital crime violation of a law derived at only by a comparison of such wording. If a girl who has not been betrothed engages in sexual relations she is not sentenced to death by the court. If she had been betrothed only by receiving the equivalent of some money, she is considered as legally married for the purpose of being sentenced to death by stoning for adultery, even though she never lived with her betrothed. This shows that the גזרה שוה is an extremely powerful exegetical tool (compare Maimonides Hilchot Ishut 1,2 and the Talmud Kritut 5,1.) [In the Talmud there it is pointed out that although the subject of פגול, faulty intentions concerning the consumption of sacrifices, is a major part of the Torah, the penalty is arrived at only by means of a גזרה שוה. Ed.] All these ways of acquiring a woman have to be witnessed by two valid witnesses in order to be legal, although if the woman wants out of the marriage she may require a bill of divorce because her status had been in doubt (Maimonides Hilchot Ishut 4,6). [Witnesses to consummation of a marriage mean witnesses who testify that both parties had secreted themselves long enough for such consummation to be capable of having taken place. Ed.]

He is then commanded to divorce her, etc. Since the verse explains that “she did not find favor in his eyes because he found in her something lecherous.” This indicates that “he is then commanded, etc.” And [also], because it states the term והיה which is an expression of certainty, i.e., a command. (Minchas Yehudah). [An alternative explanation]. Because it should have stated, “If he found in her something lecherous, he shall send her from his home.” Why is it written, “Should she not find favor in his eyes, etc.”? This indicates that the verse is teaching that you should not tell him that he can send her away even after a while, because in the meantime she might find favor in his eyes and he will not send her away. Therefore the Torah commands him to send her away [immediately]. [The text is reconstructed as follows: “Should he find in her something lecherous, she must not find favor in his eyes”]. I know what Re”m had written and I do not want to be lengthy (Nachalas Yaakov).