The Lord is using irony here: he’s not exhorting us to emulate the “sons of this world” (epitomized here by the dishonest steward) in their dishonesty, but to apply ourselves rather to laying up for ourselves treasures of in heaven with (at least) the same diligence and zeal that the “sons of this world” apply themselves to laying up for themselves treasures on earth.
Clarifying, verse 10 states, without irony,“You cannot serve God and mammon.”
Here are some comments from The Navarre Bible and Ignatius Catholic Study Bible:
From The Navarre Bible: St. Luke
16:1-8. The unfaithful steward manages to avoid falling on hard times. Of course, our Lord presumes that we realize the immorality of the man’s behavior. What he emphasizes and praises, however, is his shrewdness and effort: he tries to derive maximum material advantage from his former position as steward. In saving our soul and spreading the Kingdom of God, our Lord wants us to apply at least the same ingenuity and effort as people put into their worldly affairs to their attempts to attain some human ideal. The fact that we can count on God’s grace does not in any way exempt us from the need to employ all available legitimate human resources even if it means strenuous effort and heroic sacrifice. "What zeal people put into their earthly affairs: dreaming of honors, striving for riches, bent on sensuality! Men and women, rich and poor, old and middle-aged and young and even children: all of them alike. When you and I put the same zeal into the affairs of our souls, we will have a living and working faith. And there will be no obstacle that we cannot overcome in our apostolic works: (St. J. Escriva, The Way, 317).
Ignatius Study Bible: St. Luke
16:1-8. The parable of the Unrighteous Steward is about urgency and preparedness. About to lose his position, the steward makes use of a pressing situation to find favor with his master’s debtors and prepare for his future. Christians should take even greater care to prepare for life in the world to come. Mystically (St. Gaudentius, Sermo 18), the unrighteous steward signifies the devil, whose dominion over this world is nearing its end. Having wasted the Lord’s goods by stripping us of divine grace and friendship, he now works anxiously to make friends be deception and empty promises of forgiveness. While his ardor and foresight are worthy of imitation, his wicked and dishonest tactics are not.
16:1 a steward: A head servant who handled the business affairs of his master’s estate. Like the prodigal son, this manager wasted his master’s goods.
16:8 his prudence: The master, although cheated by the debt reduction, commends the steward for his shrewdness. He recognizes that the steward’s last-minute efforts proves successful in winning the favor of the debtors and making his financial future more secure. The unjust strategy of the steward shows that he was motivated by an entirely selfish concern for his own temporal welfare. Jesus points to the steward as both an example and a warning. As an example, the steward shows how to expend every effort in making use of our means to prepare for the future. Just as his cunning won him a comfortable living in the “houses” of his master’s debtors, so believers are challenged to make friends by almsgiving in order to be received into “eternal habitations”. As a warning, the steward is intended to characterize the attitude of the Pharisees, who have been listening to Jesus since 15:2 and who are charged with being “lovers of money” in 16:14. It is implied that the Pharisees are despising God by their devotion to mammon, i.e., they seek not eternal riches but the esteem of men and the temporal comforts of this world (16:13).
