What deceitful thing did Fix do in order to prevent Passepartout from rejoining Fogg?

Fix revealed that Fogg was a robber and offered to split the reward with Passespartout. Then..... he got him really drunk and supplied him with opium.

See here," replied Fix; "I have tracked Mr. Fogg to this place, but as yet I have failed to receive the warrant of arrest for which I sent to London. You must help me to keep him here in Hong Kong--"

"I! But I--"

"I will share with you the two thousand pounds reward offered by the Bank of England."

"Never!" replied Passepartout, who tried to rise, but fell back, exhausted in mind and body.

"Mr. Fix," he stammered, "even should what you say be true-- if my master is really the robber you are seeking for--which I deny-- I have been, am, in his service; I have seen his generosity and goodness; and I will never betray him--not for all the gold in the world. I come from a village where they don't eat that kind of bread!"

"You refuse?"

"I refuse."

"Consider that I've said nothing," said Fix; "and let us drink."

"Yes; let us drink!"

Passepartout felt himself yielding more and more to the effects of the liquor. Fix, seeing that he must, at all hazards, be separated from his master, wished to entirely overcome him. Some pipes full of opium lay upon the table. Fix slipped one into Passepartout's hand. He took it, put it between his lips, lit it, drew several puffs, and his head, becoming heavy under the influence of the narcotic, fell upon the table.