פירוש על קידושין 37:14

A father has the legal right to betroth his minor daughter, but a minor does not have the legal ability to act as an agent for an adult. So how can a father send off his own minor daughter to accept her own kiddushin? To accomplish this Rava draws an analogy to the girl sold as a slave who then becomes designated as a wife to either the master or his son. R. Yose bar R. Yehudah said that the original sale was not the money for kiddushin. When the master designates her, he must give her kiddushin money or forgive her a perutah’s worth of work. Thus the father has essentially appointed the daughter to be the agent to accept her own kiddushin. So too this can happen in a non-slave case.