A teddy bear goes into the forest at night and comes back home each night/morning with something different about him.  A missing button, mud on him (possibly a missing hat?).  There is a little boy as the teddy bear’s owner who notices these things askew.  (this is not a Fred Asche book).   Maybe the boy starts to go with the bear at the end, the boy might follow the bear into the woods after he notices that things are different about him.  
The cover had a teddy bear forward facing (wearing overalls I think) and the little boy (blonde?).  Most likely from the late 80’s early 90’s.  The book was light colored and had speckles and a frame like border about an inch or so inset from the outside edge.