Without given Test-Pbo:

• start the Editor
• place a playerunit with a backpack
• place a ammobox
• write this into the init of the box:

clearItemCargo this;
clearMagazineCargo this;
clearWeaponCargo this;
clearBackpackCargo this;

• start the mission
• place your backpack inside the box.
• aim at the box and execute "cursorTarget clearBackpackCargo" inside the debugconsole
• notice, that the box is empty but the space-indiactor still indicate that something is takeing some space inside the box

With my given Test-Pbo:

• start the mission inside the Editor
• place your backpack inside one box.
• aim at the box and execute "cursorTarget clearBackpackCargo" inside the debugconsole
• notice, that the box is empty but the space-indiactor still indicate that something is takeing some space inside the box