Mishnah 7 in our chapter deals with the case of "HaMashlish Ma'os L'Vito" - a father who deposited money with a third party for his daughter's needs, and then died before the money was used for the purpose he had in mind.
The case of the Mishnah:
A father deposits money with a shalish (a trustee) on behalf of his daughter.
The purpose of the money: to buy a field for her, or to prepare her dowry for her marriage.
The father dies.
The daughter, who has already been betrothed through kiddushin - but has not yet entered into full marriage - says to the trustee: "Ne'eman Ba'ali Alai" - my husband is trusted by me to do with the money as my father wished, and therefore hand the money over to him. (According to one understanding: hand him the money and he will do with it what is proper.)
The opinion of Rabbi Meir:
"Ya'aseh HaShalish Mah SheHushlash B'Yado - Divrei Rabbi Meir" - we do not listen to her, and the trustee may not hand the money over to the husband; he must do with it exactly what he was entrusted to do - to buy the field with it, or to provide the dowry. The Gemara explains that the basis of this ruling is the principle "it is a mitzvah to fulfill the words of the deceased," and this applies even though the father gave his instructions while he was healthy and of clear mind.
The opinion of Rabbi Yosi:
Rabbi Yosi disagrees and says: "V'Chi Einah Al HaSadeh" - suppose the trustee did what was required of him and bought the field in accordance with the father's wish, and afterwards the daughter wanted to sell it and give the proceeds to her husband - "Harei Mechurah Me'Achshav" - it is as if it were already sold. If so, there is no difference between her receiving the money after the field is bought and her receiving it beforehand, and therefore we do listen to her.
To what cases does this apply:
An adult who has been betrothed: here Rabbi Meir and Rabbi Yosi argue whether the trustee must carry out what he was entrusted to do. The Mishnah's emphasis that we are dealing with a betrothed girl is necessary, for had she already entered full marriage, even Rabbi Meir agrees that we listen to her. The father's intention was only that the trustee use the money until she marries; once she is married, all her financial affairs are bound up with her husband, and the rights to the money belong to him in any case. The question therefore exists only while she is still in the status of a betrothed woman.
A minor: even if she is already married - "the act of a minor is of no legal significance." Rabbi Yosi's argument rested on the fact that she is able to sell the field and take its proceeds, whereas a minor has no ability to sell. Therefore even according to Rabbi Yosi we do not listen to her as long as she is a minor, and the trustee must carry out the father's instructions.
In summary: Rabbi Meir and Rabbi Yosi argue about a daughter who has been betrothed and who asks that the deposited money be handed over to her husband: according to Rabbi Meir the trustee must carry out the father's instructions, based on the rule that "it is a mitzvah to fulfill the words of the deceased," while according to Rabbi Yosi we listen to her, since in any case she could sell the field that would be bought and give its proceeds to her husband. This dispute applies only to an adult, and only while she is still betrothed; regarding a married woman even Rabbi Meir agrees that we listen to her, and regarding a minor even Rabbi Yosi agrees that we do not listen to her, since the act of a minor is of no legal significance.