賭咒為誓 Sworn by Oath
小妖道：「何用？」行者道：「我與你寫個合同文書。你將這兩件裝人的寶貝換了我一件裝天的寶貝，恐人心不平，向後去日久年深，有甚反悔不便，故寫此各執為照。」小妖道：「此間又無筆墨，寫甚文書？我與你賭個咒罷。」行者道：「怎麼樣賭？」小妖道：「我兩件裝人之寶，貼換你一件裝天之寶，若有反悔，一年四季遭瘟。」行者笑道：「我是決不反悔；如有反悔，也照你四季遭瘟。」小妖道：「何用？」行者道：「我与你写个合同文书。你将这两件装人的宝贝换了我一件装天的宝贝，恐人心不平，向后去日久年深，有甚反悔不便，故写此各执为照。」小妖道：「此间又无笔墨，写甚文书？我与你赌个咒罢。」行者道：「怎么样赌？」小妖道：「我两件装人之宝，贴换你一件装天之宝，若有反悔，一年四季遭瘟。」行者笑道：「我是决不反悔；如有反悔，也照你四季遭瘟。」
逐句解 Line by line
The imps said: "What for?" Pilgrim said: "I'll draw up a contract with you.
合同文書 (hé tóng wén shū) is a contract, a written agreement -- and 合同 is still the ordinary modern Chinese word for one. Pilgrim proposes to give a completed fraud the full apparatus of commercial good faith.
"You've traded your two people-bottling treasures for my one sky-bottling treasure; lest hearts prove uneasy and, as the years wear on, there be any reneging or dispute, we'll write this out, each to keep a copy for proof."
日久年深 (rì jiǔ nián shēn), "as the days lengthen into years." 有甚反悔 (yǒu shén fǎn huǐ), "should there be any going back on it," the question-word shén. And 各執為照 (gè zhí wéi zhào), "each holding a copy as proof," 為 read wéi -- boilerplate then, and boilerplate now, since a Chinese contract still ends by stating how many counterparts exist and who keeps which.
The imps said: "There's no brush or ink here — what document can we write? Let's swear an oath instead."
寫甚文書 (xiě shén wén shū), "write what document?" -- the question-word shén. 賭個咒 (dǔ gè zhòu), "let's swear an oath instead," with the 罷 (ba) of a suggestion. Having no brush and no ink, the imps propose to make the arrangement binding by invoking supernatural punishment, which they consider the practical option.
Pilgrim said: "How shall we swear it?"
怎麼樣賭 (zěn me yàng dǔ), "sworn how?" He hands them the pen, so to speak, and lets them draft the terms of their own binding.
The imps said: "Our two people-bottling treasures, makeweight and all, for your one sky-bottling treasure — if either reneges, may he be plagued through all four seasons of the year."
貼換 (tiē huàn) is to trade with something added on to even the deal up -- the 貼 Clever-Bug offered a few passages back, when he threw in the vase to close the gap. So the count is two of theirs against one of his, the second of the two being the makeweight, and not three items in all. 遭瘟 (zāo wēn), "be struck by plague," is the standard curse called down on an oath-breaker.
Pilgrim said with a laugh: "I'll certainly never renege; and if I do, may I too be plagued through the four seasons, just as you say."
決不反悔 (jué bù fǎn huǐ), "I will absolutely not go back on it." He swears it laughing, and he is telling the exact truth: he has no intention whatever of reversing a trade this good.
Pilgrim's mock-legalism reaches its height. He proposes a formal contract, each party keeping a counterpart for proof, in case disputes should arise in years to come -- a solemn apparatus of good faith erected over a deal that has already ruined the other side. The humour is in the gravity with which he treats the imaginary future of a bargain that has no future. He is play-acting fairness for the pleasure of it.
When the imps, having neither brush nor ink, propose an oath instead, he lets them bind themselves and then swears the plague-curse back at them, laughing, and truthfully. The scene completes the con's transformation of theft into ceremony: what began as a stick across a road has become a sky-bottling wonder, a negotiated exchange and a mutual sacred vow. The imps walk away cursed, cheated, and convinced they have made the bargain of their lives.