Good point. I would raise another point here which all the Meforshim ask: Doesn't Hash-m anyway have to keep his promises irrespective of what people might say, why does he mention Avraham over here? It's possible however that the Gemara is referring to Avraham's righteousness in believing in Hash-m. The Torah says that Avraham believed Hash-m and Hash-m considered this as a Tzedakah, meaning that the merit of believing in Hash-m is itself a Tzedakah. The Ramban is puzzled by this and explains that Avraham believed that what Hash-m was doing is Tzedakah, but the Rosh explains in his commentary on the Torah that believing in Hash-m and the words of the prophets is itself Tzedakah.

The deeper meaning behind this is that Tzadik is one who fulfills his word, as Yaakov Avinu said Ve'Ansa Bi Tzidkasi, since he fulfilled his word and worked for Lavan seven years without respite. Thus Avraham believed in Hash-m because his Midah was to be a Tzadik,i.e. to fulfill what he was obligated. This is apparent in another pasuk where it says that Avraham will command his offspring to do Tzedakah and justice.

Avraham repayed his debts after returning from Egypt in order to fulfill his word. He also did not take from the spoils of war since his goal was to release Lot and not to gain riches. He insisted on fulfilling his word regarding Efron and paid him in full. Avraham saw this as emulating Hash-m who also fulfills his word, as regarding the Mabul he promised that there would not be another Mabul. This is why he believed that he would have offspring despite his age.

Therefore Hash-m says Oso Tzadik- just as he fulfilled his word faithfully, I must do Mida k'Neged Mida and fulfill my promise to him. This is not Tzedakah as we understand it-charity- but rather fulfillment of one's word.

Yoel Domb