Imputed essentially means given to you. When you read of Abraham recieving imputed righteousness for obeying God. It simply means that it was given to him for obeying God. If you remember the story it was not just a free gift but it says Abraham obeyed God. This is the righteousness that we have after salvation that pertains to our works of obedience to God. Christ righteousness is our salvation alone. Yet we are given credit for our works that we obey God and that is called Imputed. It is still "given" to us. It is as though we never could earn righteousness unless God granted this imputed grace to call us righteous because we do righteous things when we obey Him like Abraham did. Abraham was made righteous by God's command which he obeyed.