Q. If someone attends a kiddush on Shabbos and is served plated food, can he trade with his friend the portion he received, e.g. a piece of chicken for a slice of meat, or is that the same as doing business on Shabbos?

A. Rambam (H. Shabbos 23: 12) writes that the reason one may not buy, sell, rent or do business on Shabbos is that one may come to write.

Shulchan Aruch (O.H. 323: 4) permits taking food from a food seller in Shabbos, if no price is quoted. Mishna Berura (ibid. 1) rules that one should not use the term buy, sell, or pay. Mishna Berura (307: 46) adds that this may be done only when it is needed for Shabbos use.

See also Shulchan Aruch (325: 3) that permits exchanging one loan security object for others. Simchas Shabbos (28: 55) permits the substituting or swapping of permitted work shifts. However, Avnei Yoshfe (O.H. 32) prohibits the trading in Shabbos of cards that portray the photographs of Gedolim by children including their distribution amongst the youngsters.

Horav Shlomo Miler’s Shlit”a opinion is that it is permitted to trade food portions if you follow the conditions mentioned above.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a