בראשית פרק יד, כג אִם-מִחוּט וְעַד שְׂרוֹךְ-נַעַל וְאִם -אֶקַח מִכָל-אֲשֶר-לָך
מסכת סוטה דף יז/א דרש רבא בשכר שאמר אברהם אבינו אם מחוט ועד שרוך נעל זכו בניו לב' מצות חוט של תכלת ורצועה של תפלין
Rashi says Avrohom received merit since he removed himself from the stolen items. This is not understood since these items were spoils of war, and the victorious party has a right to them (how are they stolen?).
T.T. We understand that Avrohom acted in order to make a Kiddush HaShem. Thus, he could have been exempt from returning the times, but by acting out of concern for Kiddush HaShem, the items became to him like stolen items. And for this action Avrohom received merit.
Or, perhaps one could say that Avrohom chose not to benefit from spoils of a war of miracles (and not by his own might). Many miracles occurred (only 318 men or Eliezer, the sand turned into arrows, etc.) which he considered excluded the spoils from the standard transfer of ownership.
T.T. One should also be aware of the disagreement regarding Techeles. Rashi maintains that two threads of the Tzitzis are white and two are Techeles. Whereas, Rambam says three are white and only one is Techeles. Since this Midrash refers to a thread (singular) of Techeles, it seems to support Rambam’s opinion. However, Rashi could explain the singular refers to the general concept of Techeles
Also one should be aware that other Midrashim state that the merit of Avrohom for his children were the thread of Techeles and the shoe of Chalitzah (when a Yavam declines to marry the widowed sister-in-law). The shoe obviously has more of a direct connection to the verse which mentions a shoe. However, the mitzvah of Chalitzah is not a positive situation and it is difficult to refer to it as a reward.
