Yes, the novella was written specifically as a preliminary for the screenplay and even Greene notes that the movie is better. But even sketchily written Greene can be damn good and it is here.
Apart from the themes of the ruin of illusions and loyalty in post-war Vienna, the subplot involving the confusion between the two Dexters is a droll and funny commentary on the relationship between author and reader. (See Pierre Bayard's How to Talk About Books You Haven't Read
for a great take on the Dexter scene.)