Unable to set Item Defined Cost Value to a Null Value.
When setting the Item Defined Cost Value to a Null Value on Edit Mode, it will always revert to the initial value.
It is the system's design not to allow the user to set the Item Defined Cost to Null once a value has been entered. Users can change the amount value but cannot leave it blank. This is the same behavior either if the amount was edited in the UI, CSV Import or In-Line editing.
If the user needs this field to have no value, the user can put 0 as the value.