The revenge in question
Albus Dumbledore was by no means a good man, and deep down he knew it. He had many regrets: Severus Snape was one of them. At the back of his mind, though his consciousness never lingered there for too long, there was an awareness that he had failed him, refused to acknowledge his talents as a child, failed to address the abuse he was going through at home. But he had thought it was better this way; he had once tried to help another child who had looked a little like Severus, and it had been one of his greatest mistakes.
He had thought looking from afar was enough.
But when Severus had came back as a teacher, he had tried to handle him differently. To befriend him, not only because it was useful. He was not certain he had succeeded. But he knew Severus had a friend - Minerva. Severus liked most of his colleagues, and his colleagues liked him. This already was a significant victory Albus thought, looking fondly at the two Heads of House.
None of them had noticed him yet. He was still standing at the threshold of the staff room, watching quietly as Severus and Minerva sat giggling on one of the couches. Minerva was practically leaning over her friend's shoulder, looking at something he was holding in his hands.
"... so I asked my house-elf to stand silently at the back and tack pictures", Severus was saying, pointing at the muggle device in his hands. It was an old-fashioned camera, Albus recognized, wondering what they were doing with it.
Minerva laughed. "He said that? Oh, isn't he adorable. And you took the time to teach Ely how to use a camera?"
Severus pressed a button. Another picture must have appeared on the screen, for Minerva started laughing even louder.
"It was totally worth it", Severus replied, his grin broadening.
"This, Severus", Minerva said through her laughter, "is excellent blackmail material."
"I know. I keep this camera heavily warded now."
Another picture appeared. Minerva practically fell off with laughter.
"He fell!"
"You can't even begin to imagine how many times he stepped on my feet."
Wait, Albus thought. This could not be -
He stepped in, causing Minerva to jump. Severus however did not seem surprised at all, glancing at him ironically before showing the other teacher the next picture, as if nothing had happened.
Severus knew Albus was always in the staff room on this day and at this precise time, the headmaster realised.
"Minerva, you look like a child caught red-handed. What are you two doing?" he asked.
The transfiguration teacher gave Severus a startled look. But then, seeing he did not seem bothered at all, she turned towards the headmaster again.
"Severus was showing me all the trouble you went through to learn how to dance properly at the Yule Ball", she told him, pointing at the camera. "I must say, I am impressed with the result, given where you were starting from."
Albus looked half mortified, half amused. "Severus! I thought this would remain between us!"
The Potions Master pursed his lips.
"Why, did you make me sign a contract? I don't believe you did. And contracts in this school anyways, headmaster... they have little value, as you know."
"Severus!"
The Potions Master smirked. "Ah, Albus. If I were you I would laugh and tell you I'm not mocking you, but you know, your face -"
"Minerva, give it to me!"
"Certainly not", Severus said, putting the camera out of reach, "you understand this is quite valuable, headmaster. Next time you and Minerva team up to make me do something stupid, I will use this."
Albus crossed his arms, looking like a sulky child.
"I don't like your methods, my boy."
Severus' smirk widened, and he gazed proudly at the headmaster. The older man was obviously thinking of a way to get to the camera - as if someone could take something from Severus against his will.
"I told you I wanted a pay rise, Albus. Don't think anything is free when you deal with a Slytherin..."
"For a Slytherin, you behave in a very childish manner."
"And for the greatest wizard of our age, you really are too naive, Albus."