Tosafos raises the question how rava was able to force r' nasan to give tzedaka, this seems to break the rule that we don't force for a mitzvah aseh when the reward is written explicitly in the torah? Tosafos offer 3 answers. 1. He didn't really forced, just coerced with strong words. 2. The community accepted upon themselves that he would force them to give tzedaka. This implies that the din torah doesn't obligate the beis din to force for מצות עשה שמתן שכרה בצדה, but there isn't any issur in doing so, therefore the community is entitled to accept this upon themselves. 3. In addition to the aseh, there is also a l'av so the forcing was for the l'av (R"I). 4. Although Beis din isn't obligated to the point where they are punished for being lax in this area, they still maintain the right and ability to force if they choose to (Ritzva"h).
R' Elchonon (kovetz shiurim 48) asks on the third answer of Tosafos that by tzedaka we apply the principal of osek b'mitzvah patur min hamitzvah, as we see from the exemption of peruta d'rav yosef in perek eilu metzius. Since the concept of osek b'mitzvah only works to exempt from a positive mitzvah, but not from an issur, how can we apply this to tzedaka where there is a l'av? In my sefer nasiach b'chukecha (page 57) I quoted the Beis Yakov (nesivos on kesubos 49b) who asks this question. Both the beis yakov and r' elchonon answer by using the ramban in kiddushin 34a who says that sometimes a l'av is not independent and is only there to support the aseh. Therefore, in the context of עוסק במצוה פטור מן המצוה we regard tzedaka as an aseh so that one who is osek in another mitzvah is exempt from tzedaka [Alternatively, R' Elchonon says that עוסק במצוה פטור מן המצוה may apply to any mitzvah that is lost passively בשב ואל תעשה even if it is a l'av, which would explain how it applies to tzedaka].
I quoted from R' Zalman Nechemiah Goldberg Shlit"a, who asks that if tzedaka is indeed regarded as an aseh based on the ramban that the l'av is merely there to support the aseh, why does tosafos say that we would force for tzedaka due to the l'av - the l'av is not independent, it is just a support for the aseh? Rav Zalman Nechemia explains that in the context of עוסק במצוה פטור מן המצוה, the person who is osek b'mitzvah and exempt from the aseh of tzedaka is automatically exempt from the l'av which is only there to support the aseh. But in our context where he is obligated in the aseh, just that we have a rule not to force for an aseh that has the reward written next to it, the fact that there is a l'av that is also violated entitles beis din to force him so that he won't be in violation of the l'av.