There should always be a quid pro quo. 总是应该有个???(条件:Maybe I need 条件: condition)
quid pro quo: 抵偿物; 补偿物; 相等物; 代用品; 对价
"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)
I think you can translate it into a Chinese saying
天下没有免费的午餐。
The literal translation of it is there is no free lunch.