Osek B'mitzvah Patur Min Hamitzvah

Chaim B., once again, poses a lomdishe question in this post. The basic question is that Tosafos says that one may coerce someone to perform the mitzvah of tzedaka because it is not only an aseh, but a lav as well.
The problem is that the gemara says that one who is involved in the mitzva of hashavas aveidah is patur from the mitzva of tzedaka because of osek b'mitzvah patur min hamitzvah. Now, osek b'mitzvah pater min hamitzvah makes perfect sense if the mitzvah of tzedaka was only a mitzvas aseh. If, however, there is a lav as well, osek b'mitzva does not allow one to transgress a lav in the torah!
The first answer that comes to mind here is to distinguish between kum v'aseh and shev v'aal taaseh. When we say that osek b'mitzva doesn't allow one to transgress a lav, that is when the lav is b'kum v'aseh. Osek B'mitzvah is only a poter that allows you to be shev v'al taaseh. It is not, however, a mattir that allows you to actively transgress aveiros b'kum v'aseh.