且不可吃 Great King He Must Not Be Eaten
我和你都吃他一塊肉，以圖延壽長生也。」先鋒道：「大王，且不可吃。」老怪道：「既拿來，怎麼不可吃？」我和你都吃他一块肉，以图延寿长生也。」先锋道：「大王，且不可吃。」老怪道：「既拿来，怎么不可吃？」
逐句解 Line by line
You and I will each eat a piece of his flesh, and so seek to lengthen our years and live long."
長 is cháng in 長生, long life. And note the sharing: the commander offers his new vanguard an equal portion, which is the most generous thing anybody does in this cave.
The vanguard said, "Great King, he must not be eaten yet."
且 is for the moment, not yet — the imp is not forbidding the meal, only postponing it, which is the only form of refusal that works on this commander.
The old fiend said, "We have taken him. Why can he not be eaten?"
And he asks again — the fifth or sixth time in the cave that a subordinate has said something designed to be asked about. The pattern is now the joke.
The imp has just been made vanguard for a plan that delivered the monk, and his first official act is to stop the monk being eaten.
He is right, and being right is not what makes it work. What makes it work is that he postpones rather than refuses, and then waits to be asked why.