Responsa for Yevamot 151:19

Q. A had a sickly, epileptic daughter. He promised B a large dowry, and for this reason B married her. A's daughter died within the first year of her marriage, after having given birth to a child who, in turn, died before he was thirty days old. Since A's daughter left a child when she died, we decided that B was entitled to collect the dowry from A.
A. Since the child died before he was thirty days old, his viability has not been established. Therefore, his existence was of no consequence. Thus, I am greatly surprised that you considered the existence of the child a determining factor in the case. Moreover, Rashi and R. Tam hold conflicting opinions regarding the question whether a widower is entitled to collect the dowry his father-in-law had promised him (Tosaphot Ket. 47a, s. v. כתב). I am greatly surprised, therefore, that you dare decide between the two conflicting opinions; for whenever there exists a difference of opinion between authorities, we do not enforce the collection of money by either party. Your contention that since A's daughter was a sickly person B had a stronger claim on her dowry, is also baseless, for the law draws no distinction between a healthy and a sickly wife. Indeed, the takkanah of R. Tam to the effect that when a wife dies within the first year of her marriage the husband must return her dowry to the original donors, is only valid in those communities where this takkanah was accepted. My decision in this case, however, is not based on this takkanah, but on Rabbinic law which is valid everywhere.
SOURCES: Cr. 159.