Well done! You have identified that setting in this short story, at least on some levels, operates symbolically in terms of standing for an idea or abstract quality apart from its physical state. In my mind, the two settings in this short story - the ledge and the apartment, are contrasted throughout the tale. One of course, is a place of danger, of cold, and potential death, the other is a place of warmth, security and safety. It is clear that Tom's relationship with his wife is also very important to him - when he embraces her before she goes to the cinema he is almost tempted to join her:
He kissed her then and, for an instant, holding her close, smelling the perfume she had used, he was tempted to go with her; it was not actually true that he had to work tonight, though he very much wanted to.
However, he decides to follow the path of work and lets his wife go out, which of course initiates the series of events that blows the key paper out of the window and leads him to decide to go and get it. You would do well to have another read of this story and try and pick out some quotes that describes the setting of the ledge to help you in your contrast with the apartment. It is clear, however, that he is in a place of immense danger and fear, where he is not able to think, because if he thinks, he will endanger his life.
For me, it is key to analyse how Tom changes when he finally makes his way back to the apartment. He grins "in triumph" when he makes it back in, and is apparently bemused by his experience:
He shook his head wonderingly, and turned to walk toward the closet.
However, the ending makes it completely clear that his return into the apartment symbolically shows another kind of decision - that he is rejecting the life of cloak and dagger company life, where the only motivation is to get ahead of his peers, and instead opting for a more balanced approach to life, where he values his wife and marriage. This transformation is confirmed with his laughter at the loss of the key bit of paper for the second time at the end of the story.