פירוש על קידושין 118:15

R. Hisda does not understand how the betrothal of the second could just simply be removed. Poof! After all, the first person’s betrothal does not begin until after thirty days. The second person’s betrothal should be permanent.
To solve the problem, R. Joseph offers another context for Rav and Shmuel’s dispute. He also slightly modifies the words. The dispute between Rav and Shmuel is on the second clause of the mishnah, where the first man says, “From now and after thirty days” and then a second man betroths her during that thirty day period. The Talmud will now explain how their dispute maps out onto this clause.

Rav does not know whether the first betrother intended his statement as a stipulation, “If I do not change my mind within thirty days, you will be betrothed to me from now” or a retraction, i.e. “No you are not betrothed to me until after thirty days.” If it was a stipulation, then she would be married to the first man. If a retraction, then she is married to the second because the first kiddushin was never valid. Therefore, she is doubtfully betrothed to both men and will require a get from both.
Shmuel holds that the statement was a stipulation. She is doubtfully betrothed for the first thirty days but after that the second person’s betrothal is removed because the first person’s takes effect.