A group of friends agreed to take turns sponsoring their weekly dinner at a local eatery. Each week a different one of the group would pick up the tab.
Since it is inevitable that the cost will vary slightly from week to week, is there a problem of ribbis if one meal is more expensive than a previous one?
May shul members rotate a weekly sponsorship of Shalosh Seudos or “lunch and learns”?
The Rema writes (Orach Chaim 170:13) that a person should not invite his friend to a meal by stating “Come eat with me as repayment for the meal I ate with you,” as this would resemble ribbis if the second meal was more lavish. Rather he should say, “Come eat with me today, and I will eat at your home on another occasion.”
Therefore, in case #1 if the arrangement was conditional on each person honoring their part of the deal, there would be a problem of ribbis should a later meal be more expensive than the previous one.
Since this is hard to ascertain, friends should avoid making this a conditional agreement. Rather, they should specify that they do not hold each other accountable if they do not keep their part of the deal.
In case #2 one may argue that purchasing Shalosh Seudos, even when formulated as a formal arrangement, is usually done with the intention of kavod Shabbos, and not with intention of repaying the previous Shalosh Seudos.
However, rotating sponsorship of weekly “lunch and learns” where sponsors presumably are makpid on being repaid, would be prohibited. Therefore, they should clarify that their agreement is not conditional, as above.