פירוש על קידושין 38:2

According to the first opinion in this baraita, the master can designate her right up until sunset during the last day of her servitude. The idea seems to be that when the master bought her, he already paid the kiddushin money. Thus he need not forgive her any work or give her any money.
R. Yose son of R. Yehudah says that there must be time for her to work for him the value of a perutah. This is the kiddushin money, because as we have seen R. Yose son of R. Yehudah does not hold that the sale counts as kiddushin.