Is the user obligated to guarantee what he broke of the stapler, knowing that its owner was aware of its unsuitability and permitted its disposal?

There is no blame upon you for what occurred, given that your companion was pleased with it and because the damaged item is negligible. Allah the Almighty said: "There is no blame upon the blind, nor upon the lame, nor upon the sick, nor upon yourselves, if you eat from your houses… or the house of your friend." Eating involves consumption and depletion, and that which is customarily permitted is like that which is explicitly permitted. However, it is better to inform your companion about what happened to remove any doubt you may have, due to the saying of the Prophet (peace be upon him): "Consult your heart: Righteousness is that with which the soul is tranquil and the heart is at peace, and sin is that which wavers in the soul and causes uneasiness in the chest, even if people give you a fatwa (legal ruling) and advise you otherwise."