The dowry of an orphan girl who was married off while still a minor:
The Mishnah deals with an orphan girl whose father died while she was still a minor, and whose mother or brothers married her off with her consent, writing her a dowry of one hundred zuz or fifty zuz - an amount less than what the Sages established. The Mishnah rules that when she comes of age she can collect from them, from the heirs and from the estate, the amount that is truly due to her.
The reason for this: the Baraisa establishes that a daughter who is supported by her brothers after her father's death should be given ten percent of the estate as her dowry. Now, the Mishnah is speaking of a case where she married with her own knowledge and consent, and one might argue that she agreed to the smaller sum and waived her right. But she was a minor at that time - and the consent of a minor has no legal force. Therefore, once she comes of age, she collects the proper amount, ten percent.
The view of Rabbi Yehudah:
Rabbi Yehudah holds that if the father married off his first daughter during his lifetime with a particular dowry, we give the second daughter - who is now an orphan - that same amount from the estate. The basis for this is our assessment of the father's intent: not necessarily the precise sum, but an estimate based on our understanding of his thinking, what it would have been reasonable for him to give in light of what he gave to the earlier daughter. And if it is impossible to calculate what the father would have done, even Rabbi Yehudah agrees that she is given ten percent. The reason the amount is not necessarily identical is that he may have become poorer in the meantime, and the like.
The view of the Sages:
The Sages say: "Pe'amim she'adam ani u'ma'ashir, v'im hayah ashir na'aseh ani. Ela shamin es hanechasim v'nosnin lahen" - the court itself calculates what is appropriate to give, and rules according to what appears to it to be fitting, without assessing the father's intent.
The Gemara's explanation of the dispute:
The Gemara explains that the intent is not that the father actually became impoverished or wealthy, for if his situation actually changed - even Rabbi Yehudah agrees that he cannot give as he gave to the first daughter. This is also why we cannot understand Rabbi Yehudah as requiring that she be given exactly the sum of the first daughter, since circumstances may change. Rather, Rabbi Yehudah's intent is that we assess the father's thinking - what he did and what he would have done - based on his current situation.
It emerges that the Sages are referring to a change in a person's attitude: he may have become more miserly, and he may have become more generous. Therefore nothing can be determined on the basis of a conjecture about what he would have done, and it falls to the court to set the appropriate amount.