Maseches Gittin, Chapter 9, Mishnah 1. This mishnah deals with various kinds of conditions and restrictions placed upon a get: when a restriction undermines the force of the get, and when it does not affect it at all.
The mishnah opens with a case of a man who divorces his wife and says to her: "Harei at muteres l'chol adam chutz mi'ish ploni" - "You are hereby permitted to any man except for so-and-so." The get itself contains the standard text of divorce, but a restriction has been attached to it: the woman is permitted by virtue of this get even though it does not release her completely, since there remains one man whom she may not marry. In the opinion of Rabbi Eliezer, such a get is valid.
The source of the ruling - the status of a divorcee even with a partial divorce:
Rabbi Eliezer's reasoning, as the Gemara explains, is derived from the verse that forbids a kohen to marry a divorcee: "V'ishah gerushah me'ishah" - "and a woman divorced from her husband." From the wording of the verse we learn that even if the woman is divorced only from her husband, meaning that he stipulated that she is not permitted to marry any other man at all, that is enough to disqualify her from marrying a kohen. The get grants her the status of a divorcee even though it does not permit her to marry anyone. It emerges that if the husband dies, she is still considered a divorcee with regard to the laws of kehunah.
The dispute between Rabbi Eliezer and the Sages:
On this basis Rabbi Eliezer reasons: if she is considered a divorcee with regard to a kohen even when she is not permitted to any man at all, then all the more so when he permitted her to every man except for one particular man, this should be a fully valid get.
The Sages disagree and say that no proof can be drawn from there: that ruling was stated with regard to the laws of kehunah, where we are stricter with kohanim, and therefore we grant this get only a minimal status. But as a general rule, as long as the husband does not permit his wife to every man, this is not a valid get.
Distinguishing between limiting the force of the get and a condition attached to the get:
"Al menas shelo tinasi li'ploni" - "on condition that you do not marry so-and-so." If the husband said: I am giving you this get on condition that you do not marry a certain man, this is a valid get. He is not limiting the force of the get; he is merely attaching a condition. As far as the laws of gittin are concerned, and as far as marriage is concerned, she is permitted to every man; he simply does not want her to marry that particular man afterward. Such a condition does not affect the validity of the get.
"Al menas shetinasi li'ploni" - "on condition that you marry so-and-so." In the reverse case, where he said to her: you are permitted to every man on condition that you marry a certain man, even though by Torah law this is a valid get, the Sages forbade doing so. It looks as though men are exchanging wives with one another, and that is an unseemly appearance.
In summary: In this mishnah we learned that a restriction which narrows the force of the get itself - "chutz mi'ish ploni" - invalidates the get according to the Sages, even though Rabbi Eliezer validated it based on the derivation from the laws of kehunah, where a partial divorce is enough to grant the woman the status of a divorcee. In contrast, a condition that does not limit the force of the get but only stipulates her conduct afterward is valid. And in the case of "on condition that you marry so-and-so," even though the get is valid by Torah law, the Sages forbade it because of the unseemly appearance it creates.