In Genesis 15:6, Abraham was counted righteous. 
The verb employed[i]
specifies that the patriarch was accounted or reckoned as righteous;  the imputation of righteousness, rather
than an infusion of righteousness, is in view.  Many texts with the word clearly speak of imputation or
accounting,[ii]
in many others the idea of making, transforming, or infusing is evidently
impossible,[iii]
and no passages with the verb in question clearly speak of any kind of
infusion.  When Phinehas’ stand for
Jehovah and against Baalpeor was reckoned to him as righteousness (Psalm
106:31), the Divine act was certainly an accounting of Phinehas’ act as
righteous, rather than infusing goodness into or transforming his act into a
good one.  Likewise, when Nehemiah
made men treasurers because they were “counted faithful” (Nehemiah 13:13),[iv]
the accounting did not make the men faithful or infuse faithfulness into them,
but was an accounting that they were indeed faithful men.  Thus, Genesis 15:6 speaks of the legal[v]
reckoning of Abraham as righteous. 
He was reckoned righteous at the judgment bar of God, rather than in the
eyes of men, or in some other way, for Jehovah was the One who accounted the
patriarch righteous.  The opposite
of a man having righteousness accounted to him, as in Genesis 15:6, is to have
iniquity imputed (2 Samuel 19:19). 
One who has blood imputed to him is reckoned as being guilty of shedding
blood (Leviticus 17:4), while the benefit of sacrificial offering in expiation is
imputed when received in the proper manner, but not otherwise (Leviticus
7:18);  by imputation one is
reckoned as and treated as the possessor of whatever is imputed.  Thus, when Abraham was reckoned as
righteous in Genesis 15:6, his being accounted righteous, rather than his
personal acquisition of inward holiness, is in view.  Abraham, and all the righteous from the time of the first
announcement of the gospel in Genesis 3:15, acknowledged their need for
gratuiously imputed righteousness, and the Divine provision of such in the
Messiah, through their offering of animal sacrifices, as ordained by God from
the beginning (Genesis 3:20-21; 4:4); 
the blessed substitution that merited the imputation of an alien
righteousness, historically accomplished on the cross, not salvation by
personal merit, was manifestly set forth in the sacrifical types.  Similarly, David records:  “Blessed is the man unto whom the LORD imputeth not iniquity,”[vi]
the man to whom, although sinful in himself, righteousness instead of iniquity
is Divinely imputed, whose “transgression is forgiven, whose
sin is covered.”  This man is he who has received David’s
call to all nations to faith in God’s “Son,” for “blessed are all they that put their trust in him” (Psalm 2:12), even
as all are blessed who hope (Psalm 146:5) or trust in Jehovah.[vii]  Thus, “faith was reckoned to Abraham
for righteousness” (Romans 4:9)[viii]
in a legal or judicial sense. Genesis 15:6 refers solely to an imputed
righteousness.  The outward
righteousness of those imputed righteous, the outward evidential just character
manifested in them, is a consequent that follows from the receipt of imputed
righteousness, and faith, not as a meritorious instrument, but because it
embraces God and receives all freely from Him, is the root of spiritual life in
all the people of God.

The syntax of Psalm 106:31 is
very similar to that of Genesis 15:6 in its account of reckoning. Concerning
Psalm 106:31, John Gill notes:

Concerning
Psalm 106:31 “we should compare for the expression Genesis 15:6, the only
passage where it occurs, and for the subject, Deuteronomy 6:25; 24:13 . . .
Psalm 24:5.  The language does not
refer to the first justification, but to
the second, to the good works of
one already in a state of grace, by which he obtains from God, who recompenses
every one according to his works, a reward of grace, as Phinehas obtained on
the present occasion the priesthood for his family, comp. Numbers 25:13” (Comment
on Psalm 106:31, Commentary on the Psalms, vol. 3., E. W. Hengstenberg). 
That is, Phinehas’ act could only be accounted as righteous because
Phinehas’ person had already been accounted righteous through Jehovah’s
gratuitous justification;  Phinehas
had Christ as his Mediator, as one who sanctified the iniquity that otherwise
would corrupt even the holiest actions of believers and prevent them from being
acceptable in the sight of Jehovah (Exodus 28:38).

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