甚相應也 A Most Fitting Trade
伶俐蟲道：「哥啊，裝天的寶貝，與他換了罷。」精細鬼道：「他裝天的，怎肯與我裝人的相換？」伶俐蟲道：「若不肯啊，貼他這個淨瓶也罷。」行者心中暗喜道：「葫蘆換葫蘆，餘外貼淨瓶：一件換兩件，其實甚相應。」即上前扯住那伶俐蟲道：「裝天可換麼？」那怪道：「但裝天就換，不換我是你的兒子。」行者道：「也罷，也罷，我裝與你們看看。」伶俐虫道：「哥啊，装天的宝贝，与他换了罢。」精细鬼道：「他装天的，怎肯与我装人的相换？」伶俐虫道：「若不肯啊，贴他这个净瓶也罢。」行者心中暗喜道：「葫芦换葫芦，余外贴净瓶：一件换两件，其实甚相应。」即上前扯住那伶俐虫道：「装天可换么？」那怪道：「但装天就换，不换我是你的儿子。」行者道：「也罢，也罢，我装与你们看看。」
逐句解 Line by line
Clever-Bug said: "Brother — a treasure that bottles the sky! Let's trade for it."
與他換了罷 (yǔ tā huàn le ba), "let's trade for it," with the 罷 (ba) of a proposal. Greed has now taken over the negotiation on Pilgrim's behalf.
Fine-and-Keen said: "His bottles the sky — why would he trade it for ours that only bottle men?"
相換 (xiāng huàn), "swap with each other," 相 read xiāng. Fine-and-Keen is the cautious one, but note what he is cautious about: not whether the gourd is real, only whether its owner would ever agree.
Clever-Bug said: "If he won't, we'll throw in this jade vase to sweeten it."
貼 (tiē) is to add something on to even up an unequal exchange -- the makeweight you throw in to close a deal. 也罷 (yě bà), "that'll do," 罷 here the verb bà. The vase is now on the table, and Pilgrim has not asked for anything.
Pilgrim rejoiced in secret: "Gourd for gourd, and the jade vase thrown in besides — one for two! Truly a most fitting bargain."
餘外 (yú wài), "on top of that." 甚相應 (shèn xiāng yìng) is "a very good fit" -- 甚 the intensifier shèn, and 應 read yìng yet again, its third distinct job in this chapter after answering a call and answering to a name. One character for a swindler's satisfaction and a victim's doom. His own arithmetic, one for two, is worth remembering when the oath is sworn a few passages later.
He stepped forward at once, took hold of Clever-Bug, and said: "Bottle the sky, and we'll trade?"
扯住 (chě zhù), "grabbed hold of." He physically takes the imp by the sleeve before anyone can reconsider, which is a salesman's instinct and a monkey's.
The imps said: "Just bottle the sky and we'll trade — if we don't, I'm your son."
不換我是你的兒子 (bù huàn wǒ shì nǐ de ér zi), "if I don't trade, I'm your son," is a coarse oath of good faith -- claiming the junior position is the crudest way to bind yourself. 兒子 takes 兒 as the full noun ér. They mean it as a guarantee; it reads as a prophecy.
Pilgrim said: "Very well, very well — I'll bottle it for you to see."
也罷，也罷 (yě bà, yě bà), "very well, very well," 罷 again the verb bà, conceding. Pilgrim allows himself to be talked into the thing he has been steering toward since he first mentioned his gourd.
Greed finishes what Pilgrim started. Clever-Bug, dazzled, presses to trade at once; Fine-and-Keen hesitates only over terms, and his hesitation sweetens the deal, since the jade vase goes in to make up the difference. Pilgrim will get two treasures for one worthless hair, and his private glee -- one for two, a very good fit -- names the swindle exactly.
The imps then seal their own ruin with a coarse oath, and Pilgrim, feigning reluctance, allows himself to be talked into the demonstration he has been angling for from the start. This is the pivot of the whole confidence trick: everything now depends on him actually appearing to bottle the sky, which sends him, in the next passage, to petition Heaven for the loan of it.