Commentary for Kiddushin 38:3

The baraita continues with a parable. The parable talks about a person who uses an unclear betrothal formula (we will talk about this a lot later in the tractate). He says that she will be betrothed now and also after thirty days. Then, before the thirty days elapse, someone else comes and betroths her. She is betrothed to the first, for the baraita considers his betrothal as having begun “now.” The question is—what does this have to do with the slave designation? And with whose opinion does this accord?

The parable does not seem to work with R. Yose son of R. Yehudah. He holds that the sale is not a form of betrothal. She must have enough time left to work the value of a perutah. This is not like the case of betrothal that begins at two different times.
Rather, the parable is similar to the ruling of the rabbis. The betrothal begins with the sale but is only completed when he designates her. So too this man’s betrothal begins when he gives her the kiddushin and is only completed at the end of thirty days. If another man tries to betroth her in-between, she is not betrothed.