還你二萬 Chop Ten Thousand Times
老魔道：「為甚麼先砍你一刀不動，如今砍你一刀，就是兩個人？」大聖笑道：「妖怪，你切莫害怕。砍上一萬刀，還你二萬個人。」老魔道：「为甚么先砍你一刀不动，如今砍你一刀，就是两个人？」大圣笑道：「妖怪，你切莫害怕。砍上一万刀，还你二万个人。」
逐句解 Line by line
The old fiend said, "Why was it that the first cut moved you not at all, and now one cut makes two men?"
為甚麼 takes 為 at wèi — for what reason — and 甚 at shén. He is genuinely asking. Somewhere between the first cut and the second, this stopped being a duel and became a consultation.
The Great Sage laughed: "Demon, do not be alarmed."
Told not to be afraid, by the person he is trying to execute. It is the most insulting thing anybody says in the chapter.
"Chop away ten thousand times and I will give you back twenty thousand men."
還 is huán here, to give back or repay — and this is the one site in the whole chapter where the character is not the adverb hái, still. 康熙 files the repayment band under 戸關切, sounded 環, glossing it 又償也 with a citation from the 道德經. The chapter title uses the same reading in 還歸, to return.
The offer is framed as a transaction and the vocabulary makes it explicit: chop me so many and I will repay you with twice as many. Wukong has been talking contracts since he asked for paper and brush, and here he prices the deal.
The reading of 還 here is the one philological point in this chapter that had to be settled before a word was written. Fourteen of its fifteen appearances are the everyday adverb, and an unguarded reading gives that everywhere — including here, where it would turn a boast about repayment into nonsense about still having twenty thousand men.