In one of the more common form factors of the Greek Alexander Romance (by Pseudo-Callisthenes), we have this passage, located in Book II, 15:
As they began to drink more deeply, Alexander had an idea: he concealed every cup that he was given in the folds of his cloak. Those who saw him mentioned it to Darius. Darius stood up and asked him, ‘My good man, why are you concealing those cups as you dine at my table?’ Alexander thought quickly and replied, ‘Great king, whenever Alexander holds a dinner for his squadron leaders and adjutants, he gives them the cups as presents. I assumed that you would do as he does, and I supposed that this was the right thing to do.’ The Persians were quite astounded when they heard what Alexander said. Any old tale can carry its listeners, if it is told with conviction.
Alexander is later identified when the Persians notice he is not the messenger he is disguising himself as. Before that, he is even caught in the act of stealing these drinking cups. Still, perhaps this and the preceding incognito stunts in the tale do serve to strengthen the notion that Alexander can be a master of deception and quite sly inasmuch as he survives each time and charms his way out of danger when needed.
Big picture, Alexander's infiltration into the Persian ranks is to gather intelligence and deliver a message. He goes in person as per his incubation where Ammon relates of worse perils should he entrust the mission to someone under his command. But the dream did not mention the need to execute a heist of Persian drinking cups. Thus, it seems like an unnecessary risk. I could only think of a few plausible explanations:
- Alexander miscalculates how drunk the Persians were
- Alexander steals the cups out of impulse as he, himself was inebriated
- Alexander steals the cups to test the generosity of Darius
- Alexander steals the cups for a more elaborate getaway plan that ultimately didn't pan out
The cups are not featured again in the tale. Perhaps this was just a one-off rising action flourish by the authors to add a little more suspense.
Some research into what this scene may have looked like given surviving artifacts, it seems clear that the Persian drinking cups and rhytons (right-hand side) were far superior in craftsmanship to those of the Greeks (left-hand side). See image below: