Ketubot, Chapter Six, Mishnah Two
This mishnah deals with a father who promises to give his wife a large dowry and then her fiancee dies, leaving her liable for yibbum with the brother. The question is, is the father obligated to give the same amount of money to the yavam.
If a man agreed to give a fixed sum of money to his son-in-law and his son-in-law died: the Sages say that he may say I was willing to give to your brother but I am unwilling to give to you.
If a husband promises to give his daughters husband a large dowry he is legally obligated to pay that money to the son. We should note that the rabbis encouraged fathers to give their daughters generous dowries so that they would find good matches.
However, if the son-in-law dies while they are only engaged, the father-in-law is not liable to pay the same amount of money to the brother-in-law who might perform yibbum. This is true even if the daughter wants to marry the yavam. Since the fathers contract was with the other brother, he has no legal obligation to the yavam.