The Mishnah deals with a father who set aside money for his son in law - dowry money that he obligated himself to give when his daughter would enter the chuppah.
The details of the case:
Only kiddushin (betrothal) took place between the parties, and they had not yet reached the stage of nisuin (marriage).
The dowry money is given at the time of the nisuin, and therefore it had not yet been handed over to the groom.
The groom died after the betrothal and before the marriage.
The betrothed woman, the daughter of that father, now falls to yibum before the brothers of the deceased.
The father's claim:
In such a situation the father may say to the yavam: "To your brother I was willing to give such a sum as a dowry, but to you I have no interest in giving it". This is not a retraction from the deal, since the deal was not made with the yavam but with his deceased brother.
The conclusion of the Gemara is that the father may say to him: either perform chalitzah with her or marry her through yibum, but the money I am not giving you.
The novel teaching of the Beraisa:
The Gemara brings a Beraisa teaching that even if the first brother was an unlearned and coarse man and the second is a Torah scholar - where one would seemingly assume that the father would specifically want the Torah scholar - even so, the father retains the right to say: it was not with you that I made the deal but with your brother, and I have no interest in giving you this money; either marry her through yibum or perform chalitzah with her.
In summary: We have learned that dowry money is handed over at the time of the marriage, and when the groom dies after the betrothal alone, the obligation does not pass to the yavam - since the obligation was undertaken toward the deceased brother. Therefore the father may tell the yavam to perform yibum or chalitzah, but he is not obligated to give him the money, and this is so even when the yavam is superior to his brother.