From: Rabbi Moshe Sofer
To: Rabbi Wolf
The Chasam Sofer resolves two difficulties raised by his student regarding Rashi's use of a fetus (ubar) and the Rashbam's invocation of kiddushin mesuros l'bi'ah. Drawing on the Mordechai (Rabbeinu Simcha), he distinguishes that kni as v'chamor lo kana nullifies only the valid acquisition when the invalid component has absolutely zero acquisition capacity, not when it has partial capacity. Rashi chose the ubar because it has partial acquisition capacity (through inheritance), while the Rashbam used kiddushin mesuros l'bi'ah to address the case where the chamor has zero kiddushin capacity.
Source: Shu"t Chasam Sofer CM §101