פירוש על קידושין 38:8

According to R. Yose son of R. Yehudah, the initial sale is not an act of betrothal, even if the master believes that it will lead to it eventually. Therefore, if the father betroths her to someone else, she is betrothed. While this is a betrayal of the master, it is effective. The sages say that the master can designate her, even if the father tried to betroth her to someone else. They hold that the sale essentially was an act of betrothal.
The baraita draws a parable to a man who betroths a woman after thirty days. If she accepts kiddushin from someone else within those thirty days, she is betrothed to this second person.

In the parable, the master is analogous to the first man and the man to whom the father betrothed her is analogous to the second one. The parable does not work for the opinion of the sages, for they say that the master can designate her even though the father betrothed her to someone else. Thus the betrothal of the second person does not work.
Thus the parable accords with the opinion of R. Yose son of R. Yehudah. He holds that the sale is not akin to betrothal and therefore the father can betroth her to someone else. So too, a man who says “behold you are betrothed to me after thirty days” has not betrothed her until the thirty days are over.
The parable teaches us that even though when buying the girl the master did not state that his kiddushin would not begin until he designates her, this is still like a case of a man who states “behold you are betrothed to me after thirty days.” She is not betrothed until the master actually designates her and if someone else betroths her in the meanwhile, she is betrothed.