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Instructions for the Government of Armies of the United States in the Field (Lieber Code). 24 April 1863.
Section VI : Exchange of prisoners -- Flags of truce -- Flags of protection - Art. 109.
Art. 109. The exchange of prisoners of war is an act of convenience to both belligerents. If no general cartel has been concluded, it cannot be demanded by either of them. No belligerent is obliged to exchange prisoners of war.
A cartel is voidable as soon as either party has violated it.