Quid pro quo

Something for something. i.e. A favor for a favor

Quid quid latine dictum sit, altum videtur - Anything said in Latin sounds profound
Quid rides? Mutato nomine de te fabula narratur - What are you laughing at? Just change the name and the joke's on you. (Horace)
Quidnunc? Or Quid nunc? - What now? As a noun, a quidnunc is a busybody or a gossip
Quidquid agis, prudenter agas et respice finem! - Whatever you do, do cautiously, and look to the end
Quidquid id est, timeo Danaos et dona ferentes - Whatever it is, I fear the Greeks, even bearing gifts. (Vergil)
Quidquid latine dictum sit, altum videtur - Anything said in Latin sounds profound