The House with the High Wall by Arthur Gask

NOW THERE was no doubt the suggestion of Isaac Bannerstein that after all Robert might be the man who had sold the emerald had bitten deeply into David O'Shane's mind, and, when Robert came in the next morning, he gave him an even more than usually hard and intent stare. Robert could not help noticing it and could think of no other reason than that the pawnbroker was now intending to tell him he would not be wanted any more.

This idea, most disconcerting to Robert, nevertheless spurred him on to consider taking immediate steps to find out what was now actually locked up in the office safe. From the pawnbroker's continued watchfulness over it, he was sure it contained something of particular importance, and he kept on urging himself that if only he had the courage to act boldly, he could get a look inside it in the easiest way possible, as he was certain that of the two keys he had found upon the ring in the dead Fergus's pocket, one was that of the street door and the other that of the safe.