I disagree. Abraham was clearly declared righteous before Isaac was even born.
What Abraham did "after" being declared righteous proved his righteousness because his faith in God's word had a corresponding action.
Doing works in and of themselves never makes anyone righteous. We can confuse "deeds of righteousness" with righteousness itself. Righteousness only comes from Christ's work.
Genesis 15:4-6 (KJV)
[SUP]4 [/SUP] And, behold, the word of the LORD came unto him, saying, This shall not be thine heir; but he that shall come forth out of thine own bowels shall be thine heir.
[SUP]5 [/SUP] And he brought him forth abroad, and said, Look now toward heaven, and tell the stars, if thou be able to number them: and he said unto him, So shall thy seed be.
[SUP]6 [/SUP] And he believed in the LORD; and he counted it to him for righteousness.
1 Corinthians 1:30 (NASB)
[SUP]30 [/SUP] But by His doing you are in Christ Jesus, who became to us wisdom from God, and righteousness and sanctification, and redemption,
2 Corinthians 5:21 (NASB)
[SUP]21 [/SUP] He made Him who knew no sin to be sin on our behalf, so that we might become the righteousness of God in Him.