Why does the jeweler decide against having the jewels checked?

From the text:

Alone in the woods with Diana! Riding alone in the woods with Diana!
“Thousand,” he wrote, and signed it.
“Here you are,” he said.
And there opened all the flounces of the parasol, all the plumes of the peacock, the radiance of the wave, the swords and spears of Agincourt, as she rose from her chair. And the two old men and the two young men, Spencer and Marshall, Wicks and Hammond, flattened themselves behind the counter envying him as he led her through the shop to the door. And he waggled his yellow glove in their faces, and she held her honour — a Cheque for twenty thousand pounds with his signature — quite firmly in her hands.