Revision of Definition
I changed the Latin translation of this phrase from "this for that" to "something for something," which is more accurate and the preferred adaptation into English by three major English dictionaries. Here are my sources for three lexicons defining "quid pro quo":
Merriam-Webster: http://www.m-w.com/dictionary/quid%20pro%20quo
American Heritage Dictionary of the English Language http://dictionary.reference.com/search?q=quid%20pro%20quo
The New Dictionary of Cultural Literacy: http://www.bartleby.com/59/4/quidproquo.html