It is not the concrete action per se, but even if it were, the concrete action would involve a decision that entails a certain "change of heart". In that very act of relinquishing all one's wealth to follow Christ, you would have assented to and demonstrated your faithfulness to the proposition: to follow God is more important than to be wealthy, and if God were to call me to part ways with my wealth to follow God, then I should and would do it. The concrete act makes manifest the very good in question. It demonstrates and attains the temperance and rational order of the soul in question. Faith without deeds is dead.
We all like to think highly of ourselves, but if we were to find ourselves in that situation, how would we react? Most of us would behave exactly like the rich man. Few have the integrity to meet such austere moral standards. Few of us have the humility to admit that we're no better than the rich man.