Or HaChaim on Genesis 15:6/אור החיים על בראשית ט״ו:ו׳

1 והאמין בה׳ ויחשבה לו צדקה. He believed G'd who considered this as a merit for him. We can understand this as similar to the laws concerning vows. The Talmud (Rosh Hashanah 6) says that when one makes a promise to give charity to a sacred cause it is considered as binding or as effective as if one had already actually handed over the object of a vow to an ordinary person. Abraham qualified for a reward by merely expressing his belief in G'd's promise. You may well ask what practical benefit Abraham received by this reward seeing that neither Ishmael nor Isaac had been born as yet? This is why the Torah says that "he considered it an act of righteousness," i.e. also Abraham treated G'd's promise to him as if it had already been fulfilled.

2Furthermore, seeing that G'd cannot influence whether man is G'd fearing or not, i.e. "all is in the hands of heaven except the fear of heaven" (Berachot 33), Abraham could have had doubts about G'd's promise as to the quality of his offspring. The Torah therefore gives Abraham credit for not worrying how G'd could guarantee his children would be worthy. Another way of looking at Abraham's act of faith is based on the tradition (Pessikta Zutra Lech Lecha 15,5) that the world will not lack a minimum of 30 (or 36) righteous men by whose merit it continues to exist. This tradition is based on the numerical value of the word יהיה in the previous verse. Since G'd needs these 30 צדיקים, it stands to reason that He does influence these men to be pious. If so, they can certainly not claim their righteousness as their own merit as would otherwise be the case. (Compare Deut. 6,28 וצדקה תהיה לנו that performance of the commandments will be accounted as a merit for us). Abraham may have been rewarded then for not making an issue of this, although some or all of these men may have been unduly influenced to live the lives of צדיקים.