In fact, unless France said something, the Ottoman Empire itself wouldn't even know whether the troops that hadn't returned were captured or had perished in battle; in the end, they would basically be treated as though they had died in combat.
Moreover, due to the enthusiastic assistance of the Maori, the town in New Zealand had been essentially completed. Initially, there were still thousands of surviving Tunisian Guards who had been sent to labor, and they could be transferred to the Donbass at any time if needed.
Usually, the greatest barrier to developing a wilderness such as the Donbass was manpower, and Joseph happened to have a lot of free labor at his disposal.
In that era, there was no such thing as the "Geneva Convention". Prisoners of war had absolutely no rights; even if they died of exhaustion in Russia, no one would really care.