Reference for Gittin 25:1
ומה אילו עבד כהן שברח ואשת כהן שמרדה על בעלה הלא אוכלים בתרומה וזה אינו אוכל
Seeing then that a priest's slave who runs away and a priest's wife who flouts her husband can still eat of the <i>terumah</i> while this one [who is emancipated] cannot, [is it not a disadvantage to him to be emancipated]?' This was a good rejoinder, [was it not]? — Said Raba: That is the point of the answer of the Rabbis [recorded] in the Mishnah, 'BECAUSE HE IS HIS PROPERTY,' [by which they meant to say] that if the master wants he can take four <i>zuz</i> from a non-priestly Israelite [as the price of the slave], and so disqualify him wherever he is. Let us grant that R. Meir has made out his case with regard to the slave of a priest; how does he make it out with regard to the slave of an ordinary Israelite? — Said R. Samuel son of R. Isaac: [Emancipation is a disadvantage to the slave] because it disqualifies him from marrying a Gentile bondwoman. [On the contrary it is a benefit] because it qualifies him to marry a free woman? — A slave prefers a common woman; she allows him to take liberties, she is at his beck and call, she is not coy with him.