And he believed in the Lord and he counted it to him for righteousness. [Genesis 15:6 KJV]
וְהֶאֱמִן, בַּיהוָה; וַיַּחְשְׁבֶהָ לּוֹ, צְדָקָה. Genesis 15:6
The middle Hebrew word of the text appears three times in scripture, the other two being when Jacob thought Tamar was an harlot and when Eli thought Hannah was drunk.
(See the two references on Biblehub Concordance. )
On those two occasions, an estimation or an evaluation was made, in regard to appearance and on both those occasions the spectators were wrong.
But God looketh not on the outward appearance but on the heart and when God looked at the faith of Abraham, God saw something.
What he saw was - righteousness.
The Hebrew noun lacks case, only being characterised by number and grammatical gender. So there is no genitive or dative to be imagined in regard to the word 'righteousness'.
Therefore would it be correct to translate Genesis 15:6 (b) in its most basic literal form as :
... and he estimated to him - righteousness.
Edit in Response to the Accepted Answer :
The word 'reckon' as with 'account' has currency in mind so I prefer to avoid that. I take the point in regard to 'estimate'. 'Impute' I really do not like as it sounds as though something is being arbitrarily supposed. So I am staying with 'evaluate' which is the word I use for logizomai in the New Testament Koine Greek. God saw within Abraham's faith that Abraham believed that what God had promised, God would faithfully do. In other words, Abraham saw righteousness in God. And God saw his own Rightness in Abraham's faith. Thus God evaluated that to Abraham. It would be unjust not to do so.