Commentary for Kiddushin 37:10

The Talmud has mentioned R. Yose son of R. Yehudah’s opinion several times. It now explains what that opinion is. The Torah says that if the master does not designate her, then she can be redeemed. Redeeming means paying back the value of the time she has not yet served. By implication, he cannot designate her unless there is time left in her service for her to work the value of a perutah. R. Yose son of R. Yehudah reads the Torah as if it says that the master betroths (designates) her with the value of the work she has left to perform. Forgiving her from having to do the work is the kiddushin money. Hence, he must in general posit that when the master buys the slave, this does not count as kiddushin money.