降天恩免究 Heaven Remits The Inquiry
金星與行者回見玉帝道：「陷唐僧者，乃金鼻白毛老鼠成精，假設天王父子牌位。天王知之，已點兵收怪去了，望天尊赦罪。」玉帝已知此情，降天恩免究。金星与行者回见玉帝道：「陷唐僧者，乃金鼻白毛老鼠成精，假设天王父子牌位。天王知之，已点兵收怪去了，望天尊赦罪。」玉帝已知此情，降天恩免究。
逐句解 Line by line
The Gold Star and Pilgrim went back to see the Jade Emperor. "The one who trapped the Tang Monk is a gold-nosed white-haired rat turned sprite, who set up false tablets to the Heavenly King and his son.
Venus reports the truth, in one clause, and does not mention the plaint or the arrest or the blade. 父子 keeps 子 at zǐ, the formal pairing. 假設 is to set up falsely — the charge that finally sticks is against the fiend, for forging a family.
The Heavenly King knows of it and has already mustered troops and gone to take the monster. We beg the Heavenly Honoured One to pardon the offence."
天尊 is the Heavenly Honoured One, a title borrowed from Daoist liturgy. Venus asks pardon without saying whose offence he means, which is diplomacy.
The Jade Emperor already knew how the matter stood, and sent down heavenly grace remitting the inquiry.
降 is jiàng here, to send down — the other reading of the character that meant subduing three passages ago, and the two sit close enough together in this chapter to be worth pointing at. 免究 is to waive the investigation, and the four characters 已知此情 do a great deal of quiet work: the Emperor knew all along.
The Emperor knew. The novel drops that in as a subordinate clause and does not develop it, and it recolours everything — the summons, the arrest, the near-execution all happened under an eye that could have stopped them.
What is remitted is the inquiry, not any offence. The case is not decided; it is declared not worth pursuing, which is the outcome Venus has been engineering since he arrived.