Look at the concept of "fire" in the two places it appears in the story. How is it different and yet oddly unified?

Chopin's first mention of fire is a figurative one.  She describes Armand's first sight of Desiree in the following manner:

It was no wonder, when [Desiree] stood one day against the stone pillar in whose shadow she had lain asleep, eighteen years before, that Armand Aubigny riding by and seeing her there, had fallen in love with her. That was the way all the Aubignys fell in love, as if struck by a pistol shot. The wonder was that he had not loved her before; for he had known her since his father brought him home from Paris, a boy of eight, after his mother died there. The passion that awoke in him that day, when he saw her at the gate, swept along like an avalanche, or like a prairie fire, or like anything that drives headlong over all obstacles.

Chopin's second use of "fire" is, of course, at the story's surprise ending.  After Desiree has left L'Abri is disgrace, Armand commands his slaves to build a bonfire, and then he directs what is to be burned and in which order.

A graceful cradle of willow, with all its dainty furbishings, was laid upon the pyre, which had already been fed with the richness of a priceless layette. Then there were silk gowns, and velvet and satin ones added to these; laces, too, and embroideries; bonnets and gloves; for the corbeille had been of rare quality.

The last thing to go was a tiny bundle of letters; innocent little scribblings that Desiree had sent to him during the days of their espousal. There was the remnant of one back in the drawer from which he took them. But it was not Desiree's; it was part of an old letter from his mother to his father. He read it. She was thanking God for the blessing of her husband's love.

Both examples unify the story because they represent the passion of Armand and the men of his family. He falls in love with Desiree instantly, but his love is destructive; like a fire, it "destroys" all that is in its way.  Likewise, only a passionate man (or a man ruled by his emotions) would get rid of all his wife and child's reminders in such a dramatic fashion.  He watches the remnants burn just as his love for Desiree burned out.

The difference between the two scenes is that in the first, Chopin causes the reader to envision a man who is utterly taken with a woman and who will let nothing keep him from loving or being with her.  While most humans would love for another to feel this way about them, Chopin's choice of the word "fire" and "avalanche" does establish foreshadowing that a man of such passion might be dangerous. In the second scene, the literal fire establishes the end of the relationship and a revelation--that Armand's mother was part black.