Of course faith has evidence (a changed life, obedience, good works). No issue with that. But he wasn't justified at the later date. He was justified when he believed. Otherwise Abraham could not be in relationship with God. Justification is through the shedding of blood (Atonement), and accessed by faith. This is peace with God. Unless Abraham was already justified, he could not have the continuing faith to carry on with the potential sacrifice of his son Isaac. His follow through is also justification because it shows his faith is real.
James 2:21 Was not Abraham our father justified by works, when he had offered Isaac his son upon the altar?
We are justified by the faith of Christ. His works of obedience was demonstrated by His life and death.