兩個黑棗兒 Two Black Dates
又將兩個黑棗兒做一個茶鍾，著一個托盤安了對眾女說：「等我去問他，不是唐朝的便罷；若是唐朝來的，就教換茶，你卻將此茶令童兒拿出。但吃了，個個身亡，就與你報了此讎，解了煩惱也。」七女感激不盡。又将两个黑枣儿做一个茶钟，着一个托盘安了对众女说：「等我去问他，不是唐朝的便罢；若是唐朝来的，就教换茶，你却将此茶令童儿拿出。但吃了，个个身亡，就与你报了此雠，解了烦恼也。」七女感激不尽。
逐句解 Line by line
With two black dates he made up another cup, set the whole lot out on a tray, and said to the women, "Let me go and question them. If they are not from the Tang, we let it be;"
The fifth cup. Four poisoned with red dates and one clean with black, which is his own — and the black dates are the flaw in an otherwise perfect plan, because they look different. 著 is zhuó here, the lexical-verb band of a character with four readings: to take, to use. Not the zhe that keeps an action running. And 便罷 is "then let it drop", with 罷 as the verb, to have done.
"but if they are from the Tang, I shall call for the tea to be changed, and you are to have the boys carry this tea out."
教 is jiào, to have something done — he will call for a change of tea, which is an entirely ordinary hospitable gesture and also the signal. 令, to order, does the same job in the second clause. The plan runs on two words that any host might use on any afternoon.
"One drink and every one of them dies, your grievance is avenged and your vexation resolved."
但 here is not "but" — in early vernacular it means "only", "as soon as": one has only to drink. 個個, every single one. And 報讎 is to requite a wrong, the word the women used when they asked him for a 報冤之人 416 characters ago. He is quoting their request back at them as a delivery note.
The seven women were grateful beyond measure.
A small mechanical point with a real consequence. 不盡 means without end, and 盡 is raised to jìn, a falling tone — which means the 不 before it must shift to bú. The automatic tone rule runs on the reading 盡 starts with, not the one it ends with, so it has to be corrected by hand. This is the first of four such places in the chapter, and all four were predicted before a word of it was written.
The fifth cup is the plan's one weakness and he builds it in himself. He needs a cup to drink from, so he makes one up with black dates instead of red — and now there is a tray on which one cup does not match. Everything else about this scheme is meticulous. That is the loose thread.
Notice how little of the plan is violent. Ask a question, call for fresh tea, have the boys bring it out. Every step is something a host does, and the poison is carried in on the same tray as the hospitality. The Daoist quoted a proverb about the indignity of throwing a punch 309 characters ago; he has been entirely consistent since.
And then 感激不盡 — grateful beyond measure — and the scene ends on seven women thanking a man for agreeing to poison four people at tea. The narrator adds nothing. He has been withholding comment for six passages and he is not going to start now.