Justification is in effect us being given Gods righteousness..
So far you have no scripture that says Abraham was justified in Genesis 15.
He was justified in Genesis 12.
And the Scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel to Abraham beforehand, saying, “In you all the nations shall be blessed.” So then those who are of faith are blessed with believing Abraham. Galatians 3:8
He was justified in Genesis 22.
Was not Abraham our father justified by works when he offered Isaac his son on the altar?
James 2:21
blessing I will bless you, and multiplying I will multiply your descendants as the stars of the heaven and as the sand which is on the seashore; and your descendants shall possess the gate of their enemies. In your seed all the nations of the earth shall be blessed, because you have obeyed My voice.”
Genesis 22:17-18