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The Rishonim discuss a case where a father committed to honor someone with part of the bris ceremony, either as a sandak or a mohel, and then changed his mind. (See Beis Yosef YD 264)
The Maharam says that since these commitments are routinely made and kept, the commitment is enforceable in court.
Rabbeinu Yechiyel limits this to a commitment made after the baby was born.
The Radvaz holds that if the commitment was made before the baby was born, he is not obligated to honor it at all, for this would have the status of selling something that is not yet in existence. However, if he told him this after the baby was born, since these commitments are routinely made and kept, he cannot retract from his words. He concludes by saying that it is a well established principle by us that a custom is extremely significant, and one should not break it.
He proves this from our Gemora, which states: Rav Papi said in Rava’s name: A mark of identification (which the buyers would mark if they planned on buying it) on the wine barrels can effect possession (although they left them in the possession of the seller). This proves that although a proper kinyan was not performed, the making of an identifying mark can effect acquisition based on the custom of that locality.
The Rosh disagrees and says that only a standard kinyan is enforceable.
Rabbeinu Tam says that if one committed to a mohel to do his son’s bris, this has the status of a verbal commitment, and one who does not keep it is considered untrustworthy.
The Pri Yitzchak says that committing to a mohel has the status of a small gift, since the father typically cannot perform the bris, and he is simply giving the right to choose the mohel. However, committing to a sandak is a large gift, since the father himself can do that, and he is giving that right to the sandak. Since it is a large gift, a verbal commitment would not be binding.