許敗不許勝 Lose But Do Not Win
大聖有此人情，請得佛祖與眾前來，不多時，早望見城池。行者報道：「如來，那放黑氣的乃是獅駝國也。」如來道：「你先下去，到那城中，與妖精交戰，許敗不許勝。敗上來，我自收他。」大圣有此人情，请得佛祖与众前来，不多时，早望见城池。行者报道：「如来，那放黑气的乃是狮驼国也。」如来道：「你先下去，到那城中，与妖精交战，许败不许胜。败上来，我自收他。」
逐句解 Line by line
The Great Sage had this favour of them, and got the Buddha-Patriarch and his company to come; and in no long time the city walls came in sight.
人情 is a favour, and specifically the kind that puts you under obligation — the currency of Chinese social life. 請得 takes dé: succeeded in inviting.
Pilgrim reported, "Tathagata, that place giving off the black vapour is the Kingdom of Lion Camel."
The black vapour again — the same idea the poem closed on. A reader of Chinese medical and geomantic writing would take it as diagnosis: bad air over a place means something is rotten in it.
The Tathagata said, "You go down first, and give battle to the monsters in that city. You may lose, but you may not win. Come up beaten, and I will take them myself."
許敗不許勝 is six characters of tactics and the whole plan is in them: permitted to lose, not permitted to win. The Buddha needs the monsters drawn upward and out, which requires Wukong to run.
The Buddha gives Wukong an order that is also a small humiliation, and gives it without a flicker: go down, fight, and lose.
The tactics are sound. Three fiends who are winning will chase; three fiends who are losing will stay behind their walls. To get them into the open air where five hundred Arhats are waiting, somebody has to run away convincingly, and the only person who can is the one who has never done it willingly.
The favour is worth marking too. Wukong is described as having obtained this as a favour — a debt, in the ordinary social sense. He did not command the Buddha and he did not deserve it. He asked, and now he owes.