Tosafot, Ketubot 49b

“Rava compelled R. Nathan b. Ammi to contribute four hundred zuz for charity” - One may ask how he was able to compel him for tzedakah, when its reward is written next to it, as it is written “God will bless you,” and it is said in Chapter Kol Basar (Chulin 110b) that any positive commandment whose reward is written next to it, the earthly court may not compel. We can answer that he [Rava] compelled him [R. Nathan b. Ammi] with words. Alternatively, the residents of the city had agreed to give a certain amount per month, and therefore he was able to compel him [physically], as it says in Baba Batra 8b that the residents of a city are permitted to remove someone who does not follow its agreements. Another answer is that tzedakah also has two negative commandments, “Do not harden [your heart],” and “Do not shut [your hand.]” (Deuteronomy 15). [Translation by David Fried]