"Gaspar de la noche" is one of Denevi's "epiphanies" and concerns a piano instructor who is auditioning a potential student. There's an intriguing mix of elements here, which make the term epiphany seem particularly appropraite.
There's a touch of the strange in the young boy sitting on the couch, looking to all the world like a miniature adult down to the horrible suit, the boy who has been brought in by his parents in order to nurture his gift. The parents are too poor to pay for a proper teacher but recognize the child's gift. The teacher's reluctance is compounded by the strangeness, especially when the child offers to play "Gaspar de la nuit," a difficult piece which happens to be the teacher's favorite.
I didn't quite get into this story so much, with its mix of realistically sad and sort of kilter elements, but it probably hazards a reread.