If a man is divorcing his wife and says to her, “You are permitted to any man, except for Ploni [i.e. John Doe],” Rabbi Eliezer permits this [to be considered a valid divorce and stipulation], but the Sages prohibit it. What then should he do? He must take the get back from her and give it to her once more, saying, “You are permitted to any man.” But if he wrote [the stipulation] into the [get], even if he went back and erased it, it is invalid [and a new get must be issued].
Deuteronomy 24:1 allows a man to write a “bill of divorce” for his wife. The Rabbis call this document a "get." The husband has to declare in writing that the wife will henceforth be considered free to marry any other man. But what if the husband tries to interfere with her subsequent plans? Can he control her after the divorce? Rabbi Eliezer would permit this, but the Sages require the divorce to be a complete break between husband and wife.
Divorce is never simple. Even when two people equally agree that a marriage is not sustainable, they may still remain jealous of the other, and try to curtail the other's freedom. Our Mishnah shows that this phenomenon is not new.