Laylah Lav Z'man Teffillin-Shittas HaRambam

The gemara in Menachos (36b) has a discussion whether one can wear tefillin at night. L'ma'aseh there is a machlokes rishonim over how to pasken. Tosafos (among others) holds "laylah z'man tefillin" while the Rambam paskens "laylah lav z'man tefillin". However, even according to the Rambam this halacha only applies to putting on tefillin at night. If one is already wearing tefillin then one can keep it on, but this is "halacha v'ein morin kein"-we don't tell someone they can keep it on but m'ikar hadin one is not obligated to take them off.

One interesting kasha that is asked on the Rambam is from a gemara in Berachos 44B. The gemara says that the b'nei ma'arava (people in Eretz Yisroel) would make a beracha before taking off their tefillin. Tosafos says that we don't make a beracha because laylah v'Shabbos z'man tefillin and there is never any chiyuv to take off our tefillin. However, the Rambam paskens laylah lav z'man tefillin so why doesn't he say to make a beracha on removing tefillin?

There are a few answers to this question.

1) The Beis Yosef (Siman 29) and Yam Shel Shlomo (Chullin Perek 6 Siman 2) answer very simply that since according to the Rambam you can leave on your tefillin that you were already wearing, no beracha is recited upon removing them. The B'nei Ma'arava held that it is even assur to keep your tefillin on when it becomes night.

2) The Netziv in Parshas Lech Lech Perek 17:13 in the Harchev Davar has another answer. According to one shitta in the gemara the chiyuv to remove tefillin at night is learned from a posuk "v'shamrata chuka m'yamim yamima". The gemara then has a machlokes whether "v'shamarta" is an issur aseh or a lav. The Netziv says that the B'nei Ma'arava held that it is an issur aseh and therefore you make a beracha on it. The Rambam holds it is a lav and therefore you don't make a beracha.

What is this Netziv doing in the middle of Parshas Lech Lecha in the parsha of mila?

The Netziv writes that there are 2 mitzvos in mila 1) to do the mila 2) an issur aseh on the father not to cover up the mila. The beracha of "l'hachniso l'briso shel Avraham Avinu" is a beracha made on this second mitzvah. The Netziv then asks, that this mitzvah is a shev v'al ta'aseh and where do we find that one makes a beracha on an aseh that is shev v'al ta'aseh?
The answer is from the sugyah of tefillin. We see the B'nei Ma'arava made a beracha on the aseh of not putting on tefillin. Even though it is shev v'al ta'aseh we can still make a beracha.