Reference for Ketubot 138:5

<big><strong>מתני׳</strong></big> המשליש מעות לבתו והיא אומרת נאמן בעלי עלי יעשה השליש מה שהושלש בידו דברי רבי מאיר רבי יוסי אומר וכי אינה אלא שדה והיא רוצה למוכרה הרי היא מכורה מעכשיו בד"א בגדולה אבל בקטנה אין מעשה קטנה כלום:

If a man deposited a sum of money with an agent for his daugher, and [after she was betrothed] she says, “I trust my husband”, the trustee must act in accordance with the condition of his trust, the words of Rabbi Meir. Rabbi Yose says: were [the trust] a field and she wished to sell it, it would be as if it was sold immediately! To whom does this apply? To [a daughter] who has reached majority age, but in the case of a minor, there is no validity at all to the act of a minor.

Jastrow