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In exploring the topic of a convert or orphan who is married as a Na'arah, I stumbled upon a surprising piece in the Beis Shmuel:

...הואיל קידושין דאורייתא הם ונישואין אינם מדאורייתא...

...since Kiddushin is from the Torah, but Nisuin is not from the Torah...

While it's obvious that Kiddushin is from the Torah, I was surprised to see an opinion that Nisuin is not from the Torah, as I had never heard of such a thing before. Indeed, it's the Rambam's opinion that Kiddushin and Nisuin are two parts of the same Biblical mitzvah (as discussed here).

1. What is the Beis Shmuel's source for this assertion?


2. Whatever the answer to #1, how does the Rambam address it?

The Beis Shmuel does not mean to say that in general nisuin is Rabbinical. He is referring to the Mordechai which the Rema on which he is commenting refers to, which explains that since the father is dead, the nisuin in this case is Rabbinical, while the kiddushin performed before the father died is Biblical. Language of the Mordechai, תקטז:

כי אמרו אפילו ישאנה בעודה קטנה כיון שקידושיה היו קידושין דאורייתא ונשואין דרבנן דליכא אב למימסר לחופה הרי היא כביאת זנות...

[Some Rabbis] said that even if he marries her while she is still a minor, since the kiddushin was Biblical and the nissuin Rabbinical, as there is no father to transfer her to the chuppah, it will be like zenus...