Quid pro quo
Something given in return for a item of equivalent value - like tit for tat.
A Latin term meaning 'something for something' or 'this for that'. The idea is more commonly expressed in English as 'one good turn deserves another'. This has been in the language since at least 1654, as here in H. L'Estrange's The Reign of King Charles:
"One good turn deserves another."
'Quid pro quo' is in use in colloquial English but is also a legal concept in the area of trade or exchange of goods or services. A contract is said to be binding if it is quid pro quo, i.e. if it involves an exchange of goods or services for something of comparable value, usually money.
It is often used to describe corrupt practise, where favours (notably political or sexual favours) are illicitly given in exchange for cash. I.e. 'you scratch my back and I'll scratch yours'.
The 'turn' link with 'quid pro quo' was played on by the scriptwriters of the 1991 thriller movie 'The Silence of the Lambs', starring Jodie Foster and Anthony Hopkins as Clarice Starling and Dr. Hannibal Lecter:
Hannibal Lecter: Plum Island Animal Disease Research Center. Sounds charming.
Clarice Starling: That's only part of the island. There's a very, very nice beach. Terns nest there. There's beautiful...
Hannibal Lecter: Terns? If I help you, Clarice, it will be "turns" for us too. I tell you things, you tell me things. Not about this case, though. About yourself. Quid pro quo. Yes or no?