the_debt_of_time

"Quite the opposite, Sirius," Dumbledore said with a smile. "I expect you to keep
up the appearance of a rivalry in case others are watching. It would serve us well to keep
Severus's position as a double agent the highest priority."
"So basically don't fatally injure him?" Sirius asked for clarification, and Dumbledore
nodded.
Sirius finally lowered his wand. In the split second that Dumbledore's blue eyes
turned away, he sent a closed fist into a large hooked nose, knocking the supposedly defected
Death Eater to the ground.
"There," Sirius said with a triumphant smirk. "Rivalry still well established."