一命之人 Fellow Creatures of One Life
今日大有天緣，得遇師父。萬望師父大發慈悲，救我一命。若得到觀中，就是典身賣命，一定重謝深恩。」三藏聞言，認為真實，道：「先生啊，你我都是一命之人，我是僧，你是道，衣冠雖別，修行之理則同。我不救你啊，就不是出家之輩。救便救你，你卻走不得路哩。」那怪道：「立也立不起來，怎生走路？」今日大有天缘，得遇师父。万望师父大发慈悲，救我一命。若得到观中，就是典身卖命，一定重谢深恩。」三藏闻言，认为真实，道：「先生啊，你我都是一命之人，我是僧，你是道，衣冠虽别，修行之理则同。我不救你啊，就不是出家之辈。救便救你，你却走不得路哩。」那怪道：「立也立不起来，怎生走路？」
逐句解 Line by line
"Today, by great Heaven-sent chance, I have had the fortune to meet you, Master."
天緣 (tiān yuán) is a bond arranged by Heaven, the luck that is not luck. 得遇 (dé yù), "to have the good fortune to meet," takes 得 as the full dé, "to obtain."
"I beg you ten-thousandfold, Master, show your great mercy and save my life."
萬望 (wàn wàng), "I beg you ten thousand times over." 大發慈悲 (dà fā cí bēi), "bring forth your great compassion" -- and there it is, the exact virtue the demon has built his plan around. He is asking the monk to be a monk.
"If I might but reach the abbey, though I pawn my very self and sell my life, I would surely repay your deep kindness richly."
得到 (dé dào), "manage to reach," 得 as dé; 觀中, "in the abbey," 觀 still guàn. 典身賣命 (diǎn shēn mài mìng), "pawn my own body and sell my life," is a wildly extravagant promise of repayment, which is how you can tell nobody intends to repay anything. 重謝, "reward handsomely," 重 read zhòng.
Tripitaka, hearing this, took it for the truth and said: "Sir, you and I are alike creatures of one life — I a monk, you a Daoist; our robes and caps may differ, but the principle of cultivation is one and the same.
認為真實 (rèn wéi zhēn shí), "took it for the truth" -- here 為 is wéi, the verb "to deem," where a few lines back in 為何 it was the preposition wèi. Then the moral center of the episode: 一命之人, people who share one life, and 修行之理, the principle of religious practice, which Tripitaka says is the same for monk and Daoist whatever they wear on their heads. It is a genuinely large-hearted thing to say. It is also the lever.
"Were I not to save you, I'd be no true member of the religious life."
出家之輩 (chū jiā zhī bèi), "one of those who have left the household" -- the standard Chinese way of saying "a religious," and it covers monks and Daoists alike. The master reasons himself into the rescue on principle, which is exactly what the demon predicted he would do.
"Save you I will — but you cannot walk the road."
救便救你 (jiù biàn jiù nǐ), "as for saving you, I'll save you" -- 便 concedes the point before raising the difficulty. 走不得 (zǒu bù de) is the negative of possibility, 得 toneless: you cannot walk. The 哩 (li) softens it.
The fiend said: "I can't even stand up — how am I to walk?"
立不起來 (lì bù qǐ lái), "cannot get up onto my feet." 怎生 (zěn shēng), "how, in what way." He is not asking a question; he is fishing, and what he is fishing for is a ride.
The demon closes his appeal with providence and abject gratitude -- a Heaven-arranged meeting, a promise to pawn himself in repayment -- all pitched squarely at a monk's compassion. Tripitaka takes the tale for truth and answers with the moral keynote of the episode: monk and Daoist are creatures of one life, differing in dress but not in the principle they live by, so refusing rescue would be a betrayal of his own calling. The sentiment is genuinely noble. It is also exactly the lever the fiend came equipped to pull.
The exchange ends on the difficulty the demon engineered from the start: he cannot walk. Having secured the promise, he presses the helplessness -- I cannot even stand -- to draw out the next concession, and the next scene will see the master hand over his own horse. Watch 為 across these lines: wéi, "to deem," against the wèi, "for what reason," of a moment earlier. One character, two jobs, and only the sense to sort them.