Commentary for Kiddushin 116:12

Reuven sends Shimon out to betroth Rachel on his behalf. Upon seeing Rachel, Shimon decides that he himself wants to betroth her, and when he proposes betrothal, Rachel agrees. She is now betrothed to Shimon and the fact that Shimon was supposed to act as Reuven’s agent is irrelevant. Of course, we can be sure that Reuven will not be happy with Shimon and Shimon has acted shamefully with his friend (sounds like a movie plot). Nevertheless, this fact is not of legal significance.

The connection between this section and the previous one is that in both the woman under discussion is betrothed to the second man. In this case, Reuven betroths the woman but sets the betrothal to begin in thirty days. When Shimon betroths her within thirty days, she is betrothed to Shimon, because Reuven’s betrothal has not yet begun. When the thirty days are up, Reuven’s betrothal does not “kick-in”, because she is already fully betrothed to Shimon. The mishnah expresses the fact that she is fully betrothed to Shimon by stating that if she is an Israelite’s daughter and therefore prohibited to eat terumah, she is now fully betrothed to Shimon and if he is a priest she may eat terumah. Were she not fully betrothed, the mishnah would not say that she can eat terumah.