Minhag Hamedinah
Common Commercial Practice #5
Contractual Obligations
Q: In what areas are self-adopted communal practices, which are not based on regulations, binding?
A: Self-adopted communal practices are certainly binding in contractual obligations, such as employee rights, buying and selling, etc. (Rema C.M. 331:1). In contractual obligations the minhag is binding even to remove from the one in possession. Similarly, if the parties disagree regarding what was stipulated, we presume that they stipulated according to the minhag, even to require the one in possession to pay (Rebbi Akiva Eiger 331:1; C.M. 340:5).
Therefore, some write that if the party in possession claims sincerely that he did not know of the minhag (and therefore did not stipulate explicitly that he does not want to follow the practice), the minhag does not override the default Torah law (Shach 42:36).
However, if the practice is so widespread that he certainly knew of it, he cannot claim against the minhag. However, some write that even in this case we do not easily remove from the one in possession (Teshuras Shai 1:381).
