Responsa על בבא בתרא 268:6

Q. B sent A to betroth Leah as his wife on his (B's) behalf. A came to Leah's town, invited the important persons of the community, showed them proof that he acted as B's agent, and appointed witnesses, but when he came to betroth Leah as B's wife, he said: "You are hereby betrothed to me", instead of "You are hereby betrothed to B." When the witnesses objected, A said that it was a slip of the tongue, that he did not intend to betroth Leah as his wife, especially since he was married already and that he would not violate the prohibition of polygamy by Rabbenu Gershom. He, therefore, repeated the ceremony and betrothed Leah to B. Must A divorce Leah before she may marry B?
A. No, A is to be believed that he did not intend to betroth Leah as his wife, and his unintentional act is not valid.
R. Meir adds: I wrote you my opinion but I do not want you to rely on it to free Leah without a divorce until you have inquired of the Rabbis of the surrounding territory and of the Rabbis of France. If they agree with me you may accept the above decision; but if they do not agree with me, their opinion is to take precedence over mine.
SOURCES: Pr. 586, 1015; Mord. Kidd. 522, 548; Tesh. Maim. to Nashim, 3.

Q. B sent A to betroth Leah as his wife on his (B's) behalf. A came to Leah's town, invited the important persons of the community, showed them proof that he acted as B's agent, and appointed witnesses, but when he came to betroth Leah as B's wife, he said: "You are hereby betrothed to me", instead of "You are hereby betrothed to B." When the witnesses objected, A said that it was a slip of the tongue, that he did not intend to betroth Leah as his wife, especially since he was married already and that he would not violate the prohibition of polygamy by Rabbenu Gershom. He, therefore, repeated the ceremony and betrothed Leah to B. Must A divorce Leah before she may marry B?
A. No, A is to be believed that he did not intend to betroth Leah as his wife, and his unintentional act is not valid.
R. Meir adds: I wrote you my opinion but I do not want you to rely on it to free Leah without a divorce until you have inquired of the Rabbis of the surrounding territory and of the Rabbis of France. If they agree with me you may accept the above decision; but if they do not agree with me, their opinion is to take precedence over mine.
SOURCES: Pr. 586, 1015; Mord. Kidd. 522, 548; Tesh. Maim. to Nashim, 3.