Which translation is best for 'quid pro quo'?
"quid pro quo " means "a favor or advantage granted or expected in return of something". A literal translation would be 对等交换 (equal exchange)

There are some Chinese idioms similar to it

有來有往 (things go both way)

投桃报李 (repay favor with favor)

礼尚往来 (repay in kind)


  There should always be a quid pro quo. 总是应该有个 ? ? ?


总是应该有个"对等交换" (literal)

总是应该有个"礼尚往来" (idiomatic)