Ketubot 47B

שלא כתב לה אלא על מנת לכונסה. למאן דאמר לא זכה ⁠— כרבי אלעזר בן עזריה. ומאן דאמר זכה ⁠— כרבנן.

The reason is that he wrote that she would be entitled to the additional amount only on the condition that he would marry her, and since he did not marry her, she is not entitled to the extra amount. The Talmud compares the respective opinions: The one who says that the husband does not have the right to her dowry holds in accordance with the opinion of R' Elazar ben Azarya, and therefore he rules that just as a husband guarantees his wife an extra sum in her marriage contract only if they actually get married, the wife’s father also gives the dowry only on the condition that the couple marries. And the one who said that the husband does have the right to her dowry holds in accordance with the opinion of the Rabbis, because the document is fully in effect even before marriage.

לא, דכולי עלמא כרבי אלעזר בן עזריה. מאן דאמר לא זכה ⁠— כרבי אלעזר בן עזריה, ומאן דאמר זכה ⁠— עד כאן לא קאמר רבי אלעזר בן עזריה אלא מדידיה לדידה, שלא כתב לה אלא על מנת לכונסה.

The Talmud refutes this suggestion: No, everyone agrees that the halakha is in accordance with the opinion of R' Elazar ben Azarya, and the explanation is as follows: The one who said that he does not have the right to the dowry clearly holds in accordance with the opinion of R' Elazar ben Azarya, as stated above; and the one who says the husband does have the right to her dowry would distinguish between the two cases: R' Elazar ben Azarya states that the marriage contract is not fully in effect until marriage only with regard to a bestowal from him to her, i.e., the extra sum that the husband adds to her marriage contract, as he wrote it for her only on the condition that he would marry her, and he did not intend to give her anything before she became his wife.