A father who promised something to his daughter, his daughter has not acquired the gift until her husband marries her. And so too with a son, he doesn't acquire until he marries her, for all that promise [wedding presents to their children do so] with the intent that they should be married. Therefore, one who promises money to his son-in-law or his son, and he dies before he marries her, and she becomes liable for levirate marriage, the father can say to the levir: I wanted to give the gift to your brother, but I don't want to give it to you. And even if the first was an am haaretz and the second a sage, and even if the daughter wants him.
There are those who say that one who promised something to his daughter, for example like one who says during kiddushin, I will give to my daughter, the daughter has acquired it so that if she becomes a yivamah, he [the father] cannot say: I wanted to give the gift to your brother because he promised to his daughter, and the kiddushin is not reduced, through its strength, she does yibum with his brother.
One who determines to give a certain sum of money to his [future] son-in-law at the time of betrothal, and gave him the money before the marriage was finalized, and his daughter died with a son, the money should be returned to the father, for anyone who sets aside money does so with the understanding that it is on the condition of marriage. Some authorities say that this is true if he designates the sum for his son-in-law, but if he gives it to his daughter with comment for her marriage without specifying a particular man, she acquired it as a gift. Once she is betrothed, even if the betrothal is rendered irrelevant when the finacee died, the father cannot back out. Rem"a: One who promised funds to marry off an orphan, and the orphan then dies, see (Shulchan Arukh, Yoreh De'ah 253). One who designates money for his son-in-law or daughter, and then they marry, and then the daughter dies before he gave the money, some authorities say that the husband earned his claim to the money and the father must give him the entire sum that he promised. Rabbeinu Tam explained that the husband has not earned them, and we will not seize the money from the father because of this doubt so long as the son-in-law has not received it at all, even if a third party received it with both of their authority. Some authorities say that id a third party received the money, [the son-in-law] has earned his right to it. Also, if there is a guarantor [the son-in-law gets the money]. But without these measures, even if the father drew up a contract and considered it a loan to himself and she had his children, the husband has not earned the funds, and we will not seize them from the father. [Still,] if the son-in-law takes it we will not remove it from him, for he can say, "I am certain that the first view is correct." All of this is true only if the son-in-law never received the money while the daughter was alive, but if he did receive it one time, even if he then returned it to the father, [the son-in-law] has earned rights to all of it. (So it appears from the language of the edict) Rebbeinu Tam and the French Sages also declared that even if the father already gave the dowry, if the wife or husband die during the first year without viable offspring, the full sum returns to the father or his heirs. Some authorities also say that even in the second year half the dowry should be returned to the father, and that is the standard practice in these countries that follow the proclamations of the communities of Speyer, Worms and Mainz. Only the extant portion is returned; any amount that was spent or lost is exempted, provided that he did not waste it after her death on burial expenditures. Some authorities say that if the son-in-law wishes to sell property during the [first] two years, the [prospective] heirs may object for this reason. See more of these laws later on (Shulchan Arukh, Even HaEzer 118)