quid pro quo

 
Pronunciation: /ˌkwɪd prəʊ ˈkwəʊ/

noun (plural quid pro quos)

  • a favour or advantage granted in return for something: the pardon was a quid pro quo for their help in releasing hostages

Origin:

mid 16th century (denoting a medicine substituted for another): Latin, 'something for something'