They were forced to leave it with him to provide for emergencies.

Mademoiselle's attempted escape by the window showed them the necessity for it." He did not add that it was the implicit confidence they reposed in "Battista" himself that had overcome their reluctance to leave the key with the sentry.
To seal the bargain, and in earnest of all the gold to come, Garnache gave Arsenio a couple of gold louis as a loan to be repaid him when their nameless employer should pay him his fifty pistoles in Grenoble.
The sight and touch of the gold convinced Arsenio that the thing was no dream.

He told Garnache that he believed he would be on guard-duty on the night of the following Wednesday--this was Friday--and so for Wednesday next they left the execution of their plans unless, meantime, a change should be effected in the disposition of the sentries..