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The rich young man did not sin by not following Jesus's counsel to a more perfect life, but he did commit the sin of covetousness: "because he was covetous, he went away sad" (Super Evangelium S. Matthæi ch. 19).
Discussing "Whether covetousness is always a mortal sin?" (Summa Theologica II-II q. 118 a. 5 co.), St. Thomas Aquinas says covetousness is a
mortal sin when
the love of riches becomes so great as to be preferred to charity, in such wise that a man, through love of riches, fear not to act counter to the love of God and his neighbor
venial sin when
the inordinate nature of his love stops short of this, so that although he love riches too much, yet he does not prefer the love of them to the love of God, and is unwilling for the sake of riches to do anything in opposition to God or his neighbor
If the rich man embraced poverty, this would have helped him increase in charity and perfection.