I wonder what the man's excuse would have bbeen had his substitute deserted or commited some other offence?

I can understand this man's arguement. As I understand those law (depending upon either Northern or Southern version) the substitute had to be a person who otherwise would not have been subject to those conscription laws in the first place. This substitute had evidently done his duty to the fullest of his abilities. In this the man, who had provided the substitute, obligation had been fulfilled under the law. So this man's arguement was in effect against the double indemnity of the law which would again require him to serve or provide another substitue.