Is a young man considered sinful, and is his money unlawful, if a colleague of his was transferred to an unsafe location in the army instead of him, and is he responsible if any harm befell his colleague?

It is well-known that every employee has the right to request a transfer. If the transfer is processed through regular channels and your request is granted, then you are not responsible for whoever takes your place. Nothing will befall you if something undesirable happens to them, nor will it affect your salary. So, ease your mind and beware of exaggeration and being overly scrupulous.