Continuing the thought from here.
The Rambam seems to be basing his words on the Gemara [גיטין ה] that says “בפני כמה נותנו לה” – In front of how many people must he give her the גט? Apparently, it should be enough to SAY בפני נכתב in front of the בית דין for קיום שטרות, so why does it say נותנו לה?
We see from the Gemara that there is a special דין of not only saying בפני נחתם in front of a בית דין but also to hand the גט to her. That is what the Rambam writes in הלכה ה that a שליח who brought a גט says בפ”נ in front of two people [and he is the third] and then hands her the גט in their presence. So we need a בית דין for both the declaration and the handing over of the גט.
The Mishna says [כ”ט] that if the שליח became ill, he may appoint another שליח and say in front of the Beis Din בפ”נ and the second שליח brings the גט and says שליח בית דין אני.
We see that all the latter שליח has to say is שליח ב”ד אני. The reason he must say this is the subject of a machlokes. The Rashba says that it is order to PROVE that the first שליח said בפ”נ. The Meiri argues and says that he says שליח בית דין אני in order that the נתינה be בשליחות בית ומכח בית דין – He is the agent of the בית דין. That is how we explained the Rambam – the giving of the גט must be on behalf of the בית דין. By saying בפ”נ in front of בית דין it is as if they gave it themselves and he is just their שליח [as the Rambam says]. If he can’t say בפ”נ then his שליח says שליח ב”ד אני in order to establish himself as the one giving the גט on behalf of the בית דין.
That also explains why in הלכה כ”א the Rambam says that the שליח appoints the new שליח in a בית דין in order to say בפ”נ ובפ”נ in their presence. This implies that this reason is enough to explain why the שליח must be appointed in the בית דין. That dovetails nicely with שיטת הרמב”ם and Meiri that all we need here is a מעשה בית דין. So he must appoint the second שליח in the presence of a בית דין so he can say בפ”נ in front of them, thus becoming their שליח to give the גט. But according to the Rashba, he would have to appoint the second שליח in the presence of a בית דין as a proof!
That explains the Rambam in Hilchos Eidus which says that the שליח gives the גט on behalf of the בית דין. But what about the Rambam in Hilchos Geirushin which says that the שליח becomes part of the קיום הגט? Why does he split the two halachos up in two places?
Stay tuned – I hope…
[עפ”י דברי הגרמש”ש וכן ההמשך בעז”ה ועי’ בדבריו בקונטרס ביאורים סי’ י”ט להרחבה]