原文 Original text
繁體 Traditional
Reader's edition
立個合同 Let Us Draw Up A Contract
行者聞言笑道：「妖怪，你洞裡若有紙筆，取出來，與你立個合同。自今日起，就砍到明年，我也不與你當真。」行者闻言笑道：「妖怪，你洞里若有纸笔，取出来，与你立个合同。自今日起，就砍到明年，我也不与你当真。」
逐句解 Line by line
行者聞言笑道：「妖怪，你洞裡若有紙筆，取出來，與你立個合同。
行者闻言笑道：「妖怪，你洞里若有纸笔，取出来，与你立个合同。
Wukong heard this and laughed: "Demon — if you have paper and brush in that cave of yours, fetch them out and I will draw up a contract with you."
合同 is exactly what it looks like: a contract, a written agreement between parties, and the word is still the ordinary modern one. He is proposing to put a head-chopping wager in writing.
自今日起，就砍到明年，我也不與你當真。」
自今日起，就砍到明年，我也不与你当真。」
"From today, chop away until next year — I still will not hold it against you."
當 is dàng here, the take-as band — 當真 is to treat a thing as real, to take it seriously. It is the same reading as the 最當 of an earlier passage and the 停當 before that, all three of them the falling tone 康熙 files at 丁浪切.
The contract is the joke and it is a very Chinese joke. Offered a duel to the death, Wukong's response is to suggest paperwork — and the specific paperwork of a commercial agreement, drawn up in duplicate, of the kind a Ming merchant would recognise. The demon cave has been run as an office since the third gate; Wukong is now doing business with it.
The offer is also a boast, and an accurate one. He is not proposing terms; he is saying that a year of chopping would not trouble him and he is willing to sign to that effect. The next two passages test it twice.