戰上三合 Three Rounds And No More
等我捨了這老性命去與他戰上三合。三合戰得過，唐僧還是我們口裡食；戰不過，那時關了門，讓他過去罷。」遂取披掛結束了，開門前走。等我舍了这老性命去与他战上三合。三合战得过，唐僧还是我们口里食；战不过，那时关了门，让他过去罢。」遂取披挂结束了，开门前走。
逐句解 Line by line
"Let me lay down this old life of mine and go three rounds with him."
三合 is three bouts, the standard unit of a single-combat scene in this kind of fiction. He is setting a limit before he has left the room.
"If I can win through three rounds, the Tang monk is still meat in our mouths;
戰得過 puts 得 in the light de — the can-manage-to grammar. 還 is hái, still. The two halves of this sentence and the next are a near-miss for being joined: they run positive against negative in the same frame, but the second half carries a clause the first does not.
"and if I cannot, then we shut the doors and let him pass."
Word for word the surrender he ordered at the beginning of all this. The old fiend has come the whole way round and arrived back at his first instinct, which was correct then and is correct now.
So he took his harness, buckled it on, opened the door and went out ahead.
Four verbs, no adjectives, no boast. The chapter has been full of speeches and it ends this section with a demon getting dressed and walking outside.
The bargain he offers himself is oddly touching and completely rational. Three rounds, and if it goes badly, they close the doors and lose nothing but the meal. He is not fighting for the Tang monk. He is fighting for the number of rounds it takes to keep a reputation.
And the terms are precisely the ones he set out at the start of the chapter, before any of this happened: shut the doors, let him pass. Wukong's entire afternoon of disguise, capture and escape has moved the position exactly nowhere, except that a lion is now buckling on armour instead of hiding behind a bolt.