Adam Clarke on Deuteronomy 25:1

They shall justify the righteous - This is a very important passage, and is a key to several others. The word צדק tsadak is used here precisely in the same sense in which St. Paul sometimes uses the corresponding word δικαιοω, not to justify or make just, but to acquit, declare innocent, to remit punishment, or give reasons why such a one should not be punished; so here the magistrates הצדיקו hitsdiku, shall acquit, the righteous - declare him innocent, because he is found to be righteous and not wicked: so the Septuagint: και δικαιωσουσιν τον δικαιον they shall make righteous the righteous - declare him free from blame, not liable to punishment, acquitted; using the same word with St. Paul when he speaks of a sinner's justification, i. e., his acquittance from blame and punishment, because of the death of Christ in his stead.