上邦欽差 Commissioned from a Superior Court
捱到五更時候，挨城門而去，奔大路西行。就有人撞見扯住，也好折辨：只說是上邦欽差的，滅法王不敢阻滯，放我們來的。」沙僧道：「師兄處的最當，且依他行。」挨到五更时候，挨城门而去，奔大路西行。就有人撞见扯住，也好折辨：只说是上邦钦差的，灭法王不敢阻滞，放我们来的。」沙僧道：「师兄处的最当，且依他行。」
逐句解 Line by line
Drag it out to the fifth watch, press through the city gate, and make for the high road west.
The novel writes two near-identical characters here for two different words, in six characters of each other. 捱 is ái, to drag time out, to endure a wait; 挨 is āi, to press close up against something. They differ by a single stroke, and the reading follows the sense in both.
Even if somebody runs into us and lays hold of us we shall have an answer: say we are commissioned from a superior court, and the Dharma-Destroying King will not dare hold us up, but will let us through."
差 is chāi in 欽差, an officer sent under imperial warrant — not the reading that means to differ or the one that means rather. 上邦 is a superior state, one to which the local kingdom owes deference. The cover story is a bluff about rank, which is the one thing a small kingdom cannot check.
Sha Monk said: "Elder brother's arrangement is the most fitting. Let us do as he says."
當 is dàng, apt or fitting — the same reading as in the not-proper-and-safe of a few passages back, and not the dāng that means at the time of. Sha Monk gets very few lines and almost all of them are this: settling an argument by agreeing with Pilgrim.
The fallback is better than the plan. If the disguise fails they do not fight, they claim diplomatic rank — and a king who is nervous enough about legitimacy to have blessed light over his walls is exactly the king who would not risk detaining an envoy.
Sha Monk's single line does what it always does. Bajie objects, Tripitaka worries, Pilgrim schemes, and Sha Monk closes the discussion. He is the reason these conversations ever end.