V'haya eikev tishme'un es hamishpatim ha'eileh v'shamar Hashem Elokecha lecha es ha'bris v'es ha'chessed asher nishba l'avosecha.
Our parsha opens by telling us that if we observe the commandments, specifically mishpatim, then Hashem will fulfill his promise made to the avos and give us the brachos that follow.
If we are doing what we are supposed to, then shouldn't we deserve reward based on our own merits and actions, not because of the promise made to the avos? (The Sefas Emes explains that the word v'haya, which always connotes simcha, appears here because Hashem has tremendous simcha when a person earns his own reward and doesn't receive gifts based on someone else's merit.) Zechus avos is invoked when we have no other merits of our own to call on, not when we are doing everything right?
Maybe you can answer that question by way of another question. Last week's parsha ends with the pasuk, "V'shamarta es ha'mitzvah v'es ha'chukim v'es ha'mishpatim..." Meforshim are bothered by the fact that that pasuk lists off multiple categories of mitzvos -- mitzvah, chukim, mishpatim -- while the pasuk that opens our parsha refers only to the one category of mishpatim. Why the difference?
Perhaps the point of our parsha is that even if we are not exactly doing what we are supposed to -- we are only fulfilling the logical laws of mishpatim that make sense to us, but are not on target with all the mitzvos and chukim -- nonetheless, Hashem will reward us because in addition to our own actions, we have zechus avos as well.