In Prolegomena §9:
Freilich ist es auch alsdenn unbegreiflich, wie die Anschauung einer gegenwärtigen Sache mir diese sollte zu erkennen geben, wie sie an sich ist
Of course, even then it is incomprehensible how the intuition of a thing that is present should allow me to cognize it the way it is in itself
Up until now Kant refers mostly to intuitions of objects or Gegenstände, but now he suddenly changes vocabulary to thing, or Sache. I searched for object and thing in Caygill's Kant Dictionary, and there is a pretty long entry on object, explaining the differences between Ding, Objekt and Gegenstand. However, there is no mention of Sache, and the entry for thing is simply this:
thing [Ding] see APPEARANCE, OBJECT, PERSON
So, how can I best interpret Sache?