Whose son is the Pasuk referring to?
Rashi It is referring to the master passing on the Mitzvah of Yi'ud to his son, who wants to marry the Amah Ivriyah instead of him. 1
Rashi: And the money that the master paid for her serves as the Kidushin money.
What are the implications of "ve'Im li'Veno Yi'adenah"?
Yerushalmi Kidushin, 1:1: It implies that the Amah's master may be Meya'ed her to his son but not to his brother.
What if the father performs Yi'ud and not the son?
Oznayim la'Torah: The same will apply, and the Torah only writes "ke'MIshpat ha'Banos ... " by the son because it wrote by the father "asher Lo Ye'adah" with an 'Alef'.
What does the Torah mean when it writes "ke'Mishpat ha'Banos Ya'aseh lah"?
Rashi, Ramban #2 (citing Chazal), Seforno, Targum Onkelos and Targum Yonasan: With reference to the next Pasuk, it means that whichever one of them marries her is obligated 1 to give her "She'er, K'sus and Onah". 2
Ramban #1: What the Torah mean is that, if the master gives the maidservant to his son to make Yi'ud, he should provide her with a Nadan. 3
Hadar Zekenim: He gives to her a Kesuvah.
Oznayim la'Torah: Normally, one gives an Arusah twelve months to provide her with the ornaments that a Kalah wears. But concerning this poor girl whose father sold her and is too poor to afford it, the Torah took pity on her and obligated her master to take the place of her father.
Mechilta: What is there to learn from "Mishpat ha'Banos? The Torah must therefore be learning 'Mishpat ha'Banos' from Yi'ud, inasmuch as just as the Meya'ed is Chayav 'She'er, K'sus and Onah, so too, is every husband Chayav to give his wife.
Even if he marries another woman (as will be explained in the following Pasuk), how much more so if he doesn't (Ramban).
Even though he did not acquire or betroth her in the conventional manner (Seforno).
Like a man gives to his daughter, as an act of Chesed, similar to the Ha'anakah (gifts) that a man gives to his sLa'ave after setting him free (Ramban).