"Abraham is already a follower of God, someone who already has faith in him, and the context stresses Abraham’s good works and righteousness: (1) He defeated the evil kings. (2) He rescued Lot and the other captives. (3) He went to a priest of God and gave thanks for the victory. (4) He refused any reward from the wicked king of Sodom. (5) And so God himself promised to give Abraham a reward instead. (6) The fact that God is rewarding Abraham for what he has done shows this isn’t a case of a sinner coming to God and repenting so he can obtain forgiveness. It’s God rewarding a follower for faithful service. That means Abraham isn’t acquiring righteousness here for the first time. He is already righteous, as his actions have shown. Then Abraham believes the incredible promise that he will have a multitude of descendants, despite his age (cf. Rom. 4:19, Heb. 11:12), and God reckons that act of belief as a new act of righteousness on Abraham’s part."
None of the above comments really address the text of Genesis 15:6 on its own terms. There is no interaction whatsoever with the explicit language that Abraham "believed the Lord, and he counted it to him as righteousness." The "it" is a reference to his faith. His faith is the basis for receiving righteousness. That belief certainly does not preclude the obedience of Abraham. This man's trust in God and His promises was the instrumental cause of him being counted righteous, not any good works that he did. Consider the following excerpts from the Cultural Backgrounds Study Bible (New King James Version) on Romans 4:3 and Romans 4:6-8:
"In context, Gen. 15:6 (cited here) refers to dependence on God's promise. Jewish teachers developed the implications of texts they expounded; here Paul highlights the Greek term "accounted," which he uses 11 times in ch. 4 ("impute" in v. 8), to emphasize God's generosity. Many Jewish readers, by contrast, understood Abraham's faith here as one of his virtuous works that accrued merit."
"Jewish interpreters often linked texts based on a common key term; they also often expounded a reading in the Torah in connection with a reading elsewhere in Scripture. Paul here explains "credited" in Genesis 15:6 (quoted in v. 3) in light of Ps. 32:1-2 (quoted in vv. 7-8), which also speaks of what God credits to the righteous (cf. Ps. 32:5)."
There are translational differences. Many readings are legitimate. However, it is important to note that the addition of the word "act" is not present in the vast majority of translations. Moreover, it is absent from the Hebrew. The Hebrew word in Genesis 15:6 is "tsedaqah," which refers to justice or righteousness. It denotes a state of being righteous or just. It is referring to what something is. Abraham was declared righteous (something that he is), not Abraham does righteousness.
The line of reasoning in the above quoted excerpt seemingly depends on a reduction of what those promises to Abraham are: namely to some promise short of the gospel. Galatians 3:8, however, flatly calls it the gospel. Abraham looked forward to Jesus' day, and rejoiced (John 8:58). The Reformation Study Bible has this footnote on the text from Genesis 15:
"15:6 This verse provides the early core doctrine of justification by faith, not by works (Gal. 3:6–14). Abraham believed the promise of the birth of an heir from the dead (Rom. 4:17–21; Heb. 11:11, 12), and God counted Abraham to be righteous...Abraham’s justification by faith is a model of our faith in the resurrection of Jesus Christ, God’s sacrifice for sin, and God’s crediting His righteousness to us by faith (Rom. 4:22–25)."
Defining faith as a work is a huge mistake. The two are always contrasted throughout Scripture. Ephesians 2:8-9 is a well-known example. Justification is expressly an unmerited and undeserved gift of God. If faith is a work, then that passage from Ephesians becomes self-contradictory. Faith is not a meritorious cause. Thus, to say that Abraham "does faith" is nonsense.