上天遣用 Sent Out by Heaven
老龍道：「啟上大聖得知：我雖能行雨，乃上天遣用之輩。上天不差，豈敢擅自來此行雨？」老龙道：「启上大圣得知：我虽能行雨，乃上天遣用之辈。上天不差，岂敢擅自来此行雨？」
逐句解 Line by line
The old dragon said, "Let it be reported to the Great Sage for his information: I am able to make rain, true, but I am one of those sent out to do it by Heaven.
啟上 is the opening formula of a formal report to a superior, and the dragon uses it on a monkey. 得 is dé, attached to 知 to make the compound meaning to be informed. 遣用之輩, one of the dispatched sort, puts him in a category — he is staff, not a principal.
"If Heaven does not send me, how would I dare come here and make rain on my own authority?"
差 here is the dispatching sense, to send on an errand, and 擅自 means on one's own initiative — the exact word a modern Chinese civil servant would use for acting without authorisation. The dragon is not refusing. He is citing his terms of employment.
And there it is. The Dragon King of the Eastern Ocean, a being who commands storms, explains that he cannot make it rain because he has not been told to.
The joke is very dry and very deliberate. This novel's heaven is a bureaucracy — with departments, edicts, duty rosters and a chain of command — and the book has been quietly making that joke since its opening chapters, when Pilgrim's rebellion was set off by a job grading. Here the machinery works against him for once.
It is also the first hint that the drought is not a natural event. If rain requires an order and no order has come, then somebody upstairs has decided against it.