"U'tzedaka t'hiyeh lanu ki nishmor la'asos..." (6:25)
Don't we do mitzvos out of a sense of obligation, not simply to be charitable? Ramban explains that the pasuk is speaking about the concept of reward. We owe Hashem our fidelity and service for all he has done for us; we have no right to demand or expect payment or reward for mitzvos. Any reward that we receive is simply an act of charity on Hashem's part.
I would go a step further. At the Bris ben HaBesarim Hashem promised Avraham a multitude of children who would inherit Eretz Yisrael. The Torah tells us, "V'he'emin ba'Hashem v'yachshiveha lo tzedaka," Avraham believed the promise and he considered it an act of charity for him (15:6). Rashi interprets the pasuk as praising Avraham for his belief -- He, Hashem, considered him, Avraham, as performing an act of charity. Ramban takes issue with this reading. Does a Navi like Avraham who has already demonstrated his commitment to Hashem deserve praise for believing what Hashem says? The "him" the pasuk refers to, explains Ramban, is not Avraham, but rather is Hashem. Rather than view Hashem's promise as a reward for his greatness, he, Avraham viewed it as an act of charity which he did not deserve.
Hashem acts midah k'neged midah. Our Avos, as demonstrated by Avraham, did not view Hashem as obligated to reward them even when they fully earned it; they attributed Hashem's gifts to charitable love. Hopefully we carry on that legacy. Hashem in turn promises us that even though we owe him a debt of servitude, he will credit as if we acted out of a sense of charity, out of a love and desire to accept the yoke of mitzvos even if we had no obligation to do so.
On a different note: There are approx. 170 pages of Mishna Berura from the beginning of hilchos Rosh haShana until the end of hilchos Lulav. At three pages a day, you can start now and finish by about Shmini Atzeres/Simchas Torah. Just a thought.
Any suggestions on what posts should focus on for the next few weeks? Since I mentioned it, hilchos hachag is not a bad idea, but I'm open to suggestions.