The easiest way is
make an small invisible sprite object where your dragedSprite should land
create an instance variable for dragedSprite like "IsDropped = 0"
now to the events
On mouse realese - If dragedSprite overlaps invSprite then move dragedSprite to invSprite and set "IsDropped = 1"
And make a rule where you only can drag objects with instance variable "IsDropped = 0
here is a capx from a tutorial (free book) with I think the same logic
https://www.dropbox.com/s/6kiwvhoa00jdn ... .capx?dl=0
or download the book to learn more about it https://www.scirra.com/blog/172/level-z ... uct-2-book