The sum was too absurdly high (though correct). The mischief was that, unless the sum did strike her as absurdly high, it could not possibly lead by a natural stage to the remainder of the explanation.
He must contrive another path. For instance--
"There's been a mistake about the so-called death of Priam Farll."
"A mistake!" she would exclaim, all ears and eyes.
Then he would say--
"Yes. Priam Farll isn't really dead. It's his valet that's dead."
Whereupon she would burst out--
"But you were his valet!"
Whereupon he would simply shake his head, and she would steam forwards--
"Then who are you?"