If one says to his fellow: “Go and betroth so-and-so to me,” and the fellow went and betrothed her to himself, she is betrothed to this second person, though he has acted deceitfully.
Rav Gidel was negotiating to buy a piece of land, but Rav Abba bought it first. When informed, he wanted to give the land, but Rav Gidel would not take presents. It was left for students' use.
If one says to a woman, “Behold, you are betrothed to me after thirty days” (which is valid) and another man came and betrothed her meanwhile, she is betrothed to the second man.