Rif Gittin 43b {82a; 83b}

43b

{Gittin 82a}

או דילמא ע"מ הוא ובעל מנת הוא דפליג ר"א עלייהו דרבנן אבל בחוץ מודה להו דהא שייר ליה בגט
ופשט רבינא דחוץ הוא אבל בע"מ מודו ליה

Or perhaps it means "on condition," and it is by "on condition" that Rabbi Eliezer argues on the Sages, but by "to the exclusion of," he admits to them, because he is leaving off a portion in the get.
And Ravina resolved that it {in the Mishna} means "to the exclusion of," but "on condition of," they {=the Sages} admit to him {Rabbi Eliezer}.

{Gittin 83b}
{see girsaot}

אמר רבא הרי זה גיטיך ע"מ שלא תשתי יין כל ימי חיי וחייכי אין זה כריתות כל ימי חיי פלוני הרי זה כריתות

Rava said: "Behold this is your get on condition that you do not drink wine all the days of my life and your life," this is not keritut {separation, and so the get is not valid}. "All the days of the life of Ploni," this is keritut.