You show the solution in your save function, but for your benefit in your current structure it will look a bit like this
def callback():
return txtBox.get()
def save():
key = callback()
simplified, get rid of the callback function and just amend your save function to:
def save():
key = txtBox.get()
This is of course assuming you don't want the entry to become the name of the variable. That would be silliness defined.