From the Super Mario Wiki
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In the room filled with captives, Stock can be found by flipping into 3-D. By paying him 10,000 Rubees, he reveals the pass code for the safe filled with 1,000,000 Rubees (only he did not realize it is for the safe; he assumed it is Mimi's boyfriend's phone number).
Later, after Mimi leaves the mansion, he realizes information is useless except for people who need it. Then he begs the heroes to tell him how to get out of the mansion, offering to pay them for that info.