What promise does Griffin exact from Kemp?

From the text:

“Good-night,” said Kemp, and shook an invisible hand. He walked sideways to the door. Suddenly the dressing-gown walked quickly towards him. “Understand me!” said the dressing-gown. “No attempts to hamper me, or capture me! Or—”

Kemp’s face changed a little. “I thought I gave you my word,” he said.