Commentary for Kiddushin 120:13

The Mishnah seems to teach something too obvious—if he says “on condition that I give it to you from now until thirty days” she is betrothed if he gives it to her within thirty days. Why would we ever have thought otherwise?
I might have thought that he is only saying that as a way to hurry himself up and that he does not really mean to make his betrothal conditional. Therefore, the mishnah needed to teach us that this is indeed a condition. The betrothal is not effective unless he gives her the monety within thirty days.