Kiddushin 34 gives examples of mitzvos aseh which are not zman gerama which women therefore are obligated to perform: hashavas aveidah, ma’akah, shiluach hakan. Tosfos asks why it is relevant whether these mitzvos are zman gerama or not - since each of these mitzvos also is linked to a lav which women are obligated in, women have to perform the action associated with the mitzvah irrespective of the aseh.
Tosfos answers by devising cases where the aseh applies without the lav. The Ramban, however, offers a more fundemental argument. In these cases the lav does not function as an independent issur, but is the Torah’s way of strengthening the mitzvas aseh – if the aseh does not apply, the lav which goes hand in hand with it does not apply either.
The debate between Tosfos and Ramban seems to be how to understand intersecting lavim/mitzvos – do we treat each factor independently, or do the aseh and lav merge together and function as one unit either based on the criteria of the aseh (in these cases) or the lav (perhaps in other cases).
Returning to the question of the Maharatz Chiyus: how we can say oseik b’mitzvah patur min hamitzvah by tzedaka when the mitzvah carries with it two separate lavim? One might argue based on the Ramban that the lavim are not independent issurim, but only serve to strengthen the aseh. If the aseh is cancelled, the lavim do not apply either.