Sanhedrin 24b: Gambling is forbidden because a person who bets does not really mean it; he is only playing because he hopes he will win. Therefore when the loser pays the winner, the winner is stealing.
סנהדרין כד ע”ב: משחק בקוביא מאי קא עביד? ־ אמר רמי בר חמא: משום דהוה אסמכתא, ואסמכתא לא קניא.
Rabbi Yaakov Yitzchok Ruderman, rosh yeshiva of Ner Yisroel of Baltimore, was born in Dolhinov in 1901. His mother, Sheineh, was a daughter of Yaakov Puterfas, who was also R’ Yaakov Kamenetsky’s mother’s uncle and namesake.
Rav Ruderman told his nephew, R’ Ezra Neuberger, that he had called himself Yitzchok Yaakov until the age of 13, when he joined the Slabodka Yeshiva in its World War I exile in Minsk, at which time R’ Yaakov Kamenetsky was there. R’ Yaakov, who was ten years Rav Ruderman’s senior, recalled the exact name given at his bris, and told him to place Yaakov before Yitzchok.
Reb Yaakov related that when R’ Yaakov Yitzchok’s mother was with child, she was visited by a friend who told her that she could not name a son after her father, Yitzchok, because one of her husband’s two names was Yitzchok. So she asked Sheineh to name her child Yitzchok, if it would be a boy. Having already given birth to seven girls and expecting another girl, she agreed. When a boy was born and she wanted to name him for her own father, Yaakov, the question arose whether she was obligated to keep her promise to the other woman.
The Rov of Dolhinov, Rabbi Mandel Ber Shachnovitch, ruled that the baby be given two names. The order of the two names was important to R’ Yaakov because among Ashkenazim, unlike among Sephardim, the second of double names was dominant. Thus the order of the names would indicate whether the Rav of Dolhinov considered Sheineh’s promise binding, or merely proper to keep but non-obligatory.
[Since the name “Yitzchok” was second, the Rov clearly considered the promise binding. It must be that although we hold אסמכתא לא קניא – a promise made where it is possible one won’t have to keep it is not a promise – that is only when it’s obvious from the circumstances that the gambler doesn’t want to lose the bet. But here, why would anyone think she made the promise only because she didn’t think it would be a boy anyway? Therefore the promise was binding.
The Sefer Shma Garim (p. 211) quotes Rav Shach as saying that if someone decided to name a baby after his father, and then one of the gedolei hador passes away and he prefers to name after him, he must be matir neder, because a promise to do a mitzvah (of kibud av) has the status of a vow. In our case, naming after the other woman’s father would not be a mitzvah and no hataras nedarim would be necessary. Still, the promise was binding.
It’s not clear what Reb Yaakov’s source was for saying that the second name is dominant. We know that when adding a name to a sick person, the added name always comes first. But on the contrary, that might be because we want the new name to be the main one.]
Source: Making of a Godol pp. 116-117