He received it by mistake through mail-order (it's set in present time, pre 1980s?) and he took great pleasure seeing the strange behaviour going on behind closed doors. He manages to find an angle where the lens starts working again by standing on his head, whereupon he finds someone looking back at him.
He received it by mistake through mail-order
That's the part of your description that doesn't match. He bought the binoculars in a surplus store:
Eddie Quintero had bought the binoculars at Hammerman's Army & Navy Surplus of All Nations Warehouse Outlet ("Highest Quality Goods, Cash Only, All Sales Final"). He had long wanted to own a pair of really fine binoculars, because with them he hoped to see some things that otherwise he would never see. Specifically, he hoped to see girls undressing at the Chauvin Arms across the street from his furnished room.
But maybe they got there by mistake:
The binoculars were packed in a sturdy wooden box stenciled, "Section XXII, Marine Corps, Quantico, Virginia." Beneath that it read, "Restricted Issue." Just to be able to open a box like that was worth the $15.99 that Quintero had paid.
Inside the box were slabs of Styrofoam and bags of silica, and then, at last, the binoculars themselves. They were like nothing Quintero had ever seen before. The tubes were square rather than round, and there were various incomprehensible scales engraved on them. There was a tag on them which read, "Experimental. Not To Be Removed from the Testing Room."
(it's set in present time, pre 1980s?)
The owner discovers he can spy on neighbours' apartments through it,
He was looking across the street at the mammoth of the Chauvin Arms. The view was exceptionally sharp and clear: he felt that he was standing about ten feet away from the exterior of the building. He scanned the nearest apartment windows quickly, but nothing was going on. It was a hot Saturday afternoon in July, and Quintero supposed that all the girls had gone to the beach.
He turned the focus knob, and he had the sensation that he was moving, a disembodied eye riding the front of a zoom lens, closer to the apartment wall, five feet away, then one foot away and he could see little flaws in the white concrete front and pit marks on the anodized aluminum window frames. He paused to admire this unusual view, and then turned the knob again very gently. The wall loomed huge in front of him, and then suddenly he had gone completely through it and was standing inside an apartment.
and he took great pleasure seeing the strange behaviour going on behind closed doors.
Yes, he observes some very weird and inexplicable scenes.
but he drops and breaks it
Not exactly. He doesn't drop them, they already had a loose part rattling around inside when he took them out of the box. From time to time the loose part falls into place and then the binoculars work:
He looked through the glasses again, saw nothing, and began to shake and manipulate them. He could hear the loose part rolling and tumbling around, but the lenses remained dark. He kept on manipulating them, eager to see the next wonder.
The part suddenly fell into place. Taking no chances this time, Quintero put the glasses down on his carpeted floor. He lay down beside them, put his head to one side, and tried to look through one eyepiece. But the angle was wrong and he could see nothing.
He manages to find an angle where the lens starts working again by standing on his head,
He started to lift the glasses gently, but the part moved a little and he put them down carefully. Light was still shining through the lenses, but no matter how he turned and twisted his head, he could not get lined up with the eyepiece.
He thought about it for a moment, and saw only one way out of his difficulty. He stood up, straddled the glasses, and bent down with his head upside down. Now he could see through the eyepieces, but he couldn't maintain the posture. He straightened up and did some more thinking.
He saw what he had to do. He took off his shoes, straddled the binoculars again and performed a headstand. He had to do this several times before his head was positioned correctly in front of the eyepieces. He propped his feet against the wall and managed to get into a stable position.
whereupon he finds someone looking back at him.
Yes, he could see! He was looking into a dreary furnished room. Within that room he saw a thin, potbellied man in his thirties, stripped to the waist, standing on his head with his stockinged feet pressed against the wall, looking upside down into a pair of binoculars that lay on the floor and were aimed at a wall.
It took him a moment to realize that the binoculars were showing him himself.
He sat down on the floor, suddenly frightened. For he realized that he was only another performer in humanity's great circus, and he had just done one of his acts, just like the others. But who was watching? Who was the real observer?
He turned the binoculars around, and looked through the object-lenses. He saw a pair of eyes, and he thought they were his own—until one of them slowly winked at him.