At weddings, the mesader kedushin asks 2 witnesses to sign the ketubah for the chassan. Why is this allowed? Isn’t there a gemara that says that one can’t appoint a shliach to get witnesses to sign a document? Shouldn’t the chasan himself have to ask the witnesses to sign?
The question is a good question.
It is true that the chasan himself has to ask the witnesses to sign, as ruled in Shulchan Aruch (Choshen Mishpat 39:2). There are two possible solutions that take care of this issue under the chuppah (the second being more watertight than the first):
1. It is possible that because the chasan knows that the mesader kiddushin arranges the signature of the witnesses, and furthermore, the signatories are usually the witnesses to the kiddushin itself, whom the chasan specifies, it is considered as if the chasan actually instructs them “kesivu vechismu” (write and sign). This solution, however, is problematic in view of the ruling of Shulchan Aruch (39:11, according to one opinion) whereby the witnesses have to write that they were instructed to sign.
2. A kinyan is made with the chasan concerning the kesubah, and where a kinyan is made, the arrangement can be written as a shtar without further instruction (Shulchan Aruch 39:3). Rema (Even Ha-Ezer 66:1) writes that the witnesses should not sign the kesubah until the kinyan has been made, which takes care of the issue of instructing them to sign.