Responsa for Yevamot 73:21

Q. A, of priestly lineage, married a nursing widow, Leah. Before the marriage took place, Leah, following the advice of Rabbi Jacob (of Cracow), gave her infant to a wet-nurse who made a vow Al Daat Rabbim (the interpretation of said vow to lodge with an undetermined number of people, and, therefore, incapable of annulment) not to resign her position till the infant be of age to be weaned (two years). Many have disputed the efficacy of this procedure, and gave their opinions that A must divorce Leah. Rabbi Jacob pleaded that A be not required to divorce Leah since A, being of priestly lineage, would not be able to remarry her after the two year period of nursing be over.
A. A person who married a nursing widow must divorce her, even though he was of priestly lineage and consequently could not remarry her, once he divorced her. The wet-nurse's vow is of no avail since her husband could annul it even though it was made Al Daat Rabbim.
SOURCES: Pr. 864; Tesh. Maim. to Nashim, 24; cf. R. Asher, Responsa, 53, 2; Tur Eben Haezer 13.