Masechet Kiddushin, Perek Gimel, Mishnah He. The Mishnah says, “If a man gets engaged to a woman, and afterwards he says, ‘I thought she was the daughter of a Kohen (priest), but now I’m finding out that she was the daughter of a Levi (Levite).’ Or, ‘I thought she was the daughter of a Levi, but now I’m finding out that she’s the daughter of a Kohen.’ Or, ‘I thought she was a poor girl, and now I’m finding out that she’s a rich girl.’ Or, ‘I thought she was a rich girl, and now I’m finding out that she was a poor girl.’” In these cases, Harei zo mekudeshet—in these cases, she’s engaged—because she did not mislead him. Even though he says that he wouldn’t have married her if he had known differently, it was his responsibility to look into it and find out what the situation was. She never misled him. He made the assumption. She didn’t fill him in on the details. But it was his assumption, and therefore the Kiddushin (engagement) is done. It can’t be undone, because a person’s unspoken thoughts are disregarded. Even though he says, “In my mind, I wouldn’t have married her if I had known that she was different,” it doesn’t matter. He should have looked into it. He should have had a conversation with her if he wanted to. She did nothing to mislead him.
A non-Jewish man tells a Jewish woman, “You’re engaged to me after I convert to Judaism,” and hands her the money for the Kiddushin before he converts. Or a Jewish man tells a non-Jewish woman, “You are engaged to me after you convert to Judaism,” and hands her the money. Or a Canaanite slave tells a Jewish woman, “You’re engaged to me after I’m freed from my slavery.” Or a Jewish man tells a Canaanite slave woman, “You’re engaged to me after you’re freed from slavery.” Or a Jewish man tells a married woman, “You’re engaged to me after your husband dies.” Or a Jewish man tells his wife’s sister, “You’re engaged to me after your sister dies.” That’s the only permissible way. Or a Jewish man tells a woman who was waiting for Yibum (levirate marriage) from somebody else, “After you do Halitza (levirate release) with that other man, I’ll get married to you.”
In all of these cases, they’re not engaged. Even after that change happens—for example, even after the woman becomes Jewish—they’re not engaged. A person cannot do Kiddushin with somebody. If the Kiddushin isn’t possible to take effect right away, if the woman wasn’t Jewish, he can’t do Kiddushin with her, even if she’s going to convert later on. Therefore, the Kiddushin in all of these cases is invalid.
V'chen, ha'omer l'chavero, im yalda ishtecha nekevah, harei hi mekudeshet li. Like, if a man tells his friend, “If your wife gives birth to a female, that baby is going to be engaged to me.” Then he gives the man money for the Kiddushin. Let’s say the wife then has a girl. We say, Ena mekudeshet. The child is not engaged. Davar shelo ba la'olam. Baby wasn’t in existence yet. Therefore, when the act of Kiddushin was performed, there was no one to do the Kiddushin with. It couldn’t take effect at that time, because there was no baby there. Therefore, we say it’s invalid.
Im hayetah eshet chavero, me'uberet. Ve'hukar ubara, devarav kayamin. But if you have a situation where the friend’s wife was pregnant, and it was noticeable—everyone knew that the fetus was there—then the words of the man who tried to get engaged are effective. Ve'im yalda nekevah mekudeshet. And if the child is born and it’s a girl, then they are engaged. In this situation, the baby is there. You could tell. It exists. Therefore, we look at the engagement as something that is effective.