Instead of attaching, use "link", you can unlink at the moment you want to drop it.
A bit tricky to make the "drop" look good, make sure to set a keyframe in transform before unlinking.
Then you will have to look how fast your endkey (prop bottom) comes, maybe use "end in a bounce" transition key...


Toystorylab on Vimeo :     Crassitudes (my "Alter Ego") on Youtube: 

If you want to use physics for the drop (often a more realistic motion especially for the hitting the ground part) , the manual describes how to use link and change the state of the object: 

https://manual.reallusion.com/iClone_7/ENU/Content/iClone_7/Pro_7.4/20_Physics/Mixing_State_Keys_with_Linkage_Feature.htm