Kesubos 49b tells us that Rava coerced R’ Nasan bar Ami to pledge a certain sum to tzedaka. Tosfos asks: how did Rava have the right to coerce fulfillment of the mitzvah of tzedaka? Chulin 110 tells us that Bais Din cannot coerce someone to perform a mitzvas aseh which the Torah states an explicit reward for performing [it is as if the Torah designated the reward and only the reward be used as an incentive]. The Torah explicitly promises bracha for tzedaka, which would seem to exclude coercion.
Tosfos offers three answers:
1) The gemara in Chulin excludes coercion by force; Rava used “verbal coercion” to talk Rav Nasan into it.
2) The gemara in Chullin refers to voluntary pledges of tzedaka, but R’ Nasan lived in a city where there was a contractual agreement among all members to contribute a set monthly payment.
3) Tzedaka is different than other mitzvos in that there are two lavim that go with it – lo t’ametz and lo tikpotz. Coercion can be applied to force fulfillment of lavim.
4) A fourth answer not given by Tosfos but mentioned by other Rishonim: the gemara in Chulin means that Bais Din in these cases is not forced to coerce obedience, but it does not mean that Bais Din is prevented from doing so.
The Maharatz Chiyus in a number of places (e.g. Ateres Tzvi, Shu”t siman 13) questions Tosfos’ third answer. Bava Kama (56) tell us that someone watching/holding a lost article until its owner claims it has the status of a shomeir sachar because the object’s guardian receives the “payment” of being exempt from the mitzvah of tzedaka while watching the object – oseik b’mitzvah patur min hamitzvah. Oseik b’mitzvah patur min hamitzvah is an exemption from mitzvos aseh, but not lavim – e.g. travelers en route to perform a mitzvah are exempt from the mitzvah of sukkah (Sukkah 25), but cannot eat in a treif restaurant. If Tosfos is correct that the mitzvah of tzedaka carries with it two lavaim as well as the mitzvas aseh, how can oseik b’mitzvah exempt the shomeir from the lavim of tzedaka? How are those lavim different than the lav of eating tarfus?
There are a number of ways to approach answering this kashe…