How did the man repay David for finding his glasses when David refused to accept money?

From the text:

David stepped slowly forward. "Here they are," he said, and added hesitantly, "Sir." The man put the glasses on and smiled again, and David felt sure he was not one of /hem. He had quite a different look about him: David could not imaginehim striking or shooting anyone. He now felt in his pocket, took something out and offered it to David. A coin lay in his hand. "You must have something for your trouble," he said.