難解難分 Neither Loosed Nor Parted
這大聖與那牛王鬥經百十回合，不分勝負。正在難解難分之際，只聽得山峰上有人叫道：「牛爺爺，我大王多多拜上，幸賜早臨，好安座也。」牛王聞說，使混鐵棍支住金箍棒，叫道：「猢猻，你且住了，等我去一個朋友家赴會來者。」这大圣与那牛王斗经百十回合，不分胜负。正在难解难分之际，只听得山峰上有人叫道：「牛爷爷，我大王多多拜上，幸赐早临，好安座也。」牛王闻说，使混铁棍支住金箍棒，叫道：「猢狲，你且住了，等我去一个朋友家赴会来者。」
逐句解 Line by line
The Great Sage and the Bull King fought a hundred bouts and more, and no winner could be told between them.
不分勝負 keeps 分 at fēn, to divide or tell apart, so 不 stays bù in front of it.
Just at the point where they could be neither loosed nor parted, a voice was heard calling from the mountain peak: "Lord Bull! My great king sends his most respectful greetings, and begs the favour of your early coming, that the seats may be duly set."
難解難分 is a fixed four-character phrase for a deadlock nobody can prise apart — still in daily use, and often of an argument rather than a fight. 只聽得 takes dé. And 好安座 — so the seats may be properly set — is the language of a banquet steward. Hearing it arrive in the middle of a duel is the whole joke.
Hearing this, the Bull King used his wrought-iron staff to hold off the gold-hooped cudgel and called out, "Ape, hold a while. Let me go to a friend's house and attend the gathering, and I will be back."
支住 is to prop or fend off: he does not disengage, he braces. 來者 at the end is the colloquial future, and it makes the return a promise.
The duel is broken off by a dinner invitation, and the novel makes no pretence that this is anything but comic. A hundred bouts are fought to no result; a servant calls down from a peak that the seats are ready; and the Bull Demon King stops fighting to go to a party. The messenger's language is elaborately polite — most respectful greetings, the favour of an early arrival — and it is delivered over the heads of two men trying to kill each other in mid-air.
Structurally the interruption does what the poem said could not be done any other way. The fight was declared undecidable in verse one passage ago; something outside it must therefore end it, and the novel supplies a banquet. What looks like a digression is the plot's only available exit, and the chapter's whole second half is built out of where that banquet is and what the Great Sage does about it.
The Bull King's manner in breaking off is worth noting. He does not concede, does not claim victory, and does not let his opponent go — he fends the cudgel off, tells the ape to wait, and promises to come back. He is treating a duel to the death as an appointment that can be rescheduled, which is of a piece with a man who put on full plate to go and look at a visitor.