How do form "You can borrow the book once [it's returned / C returns it]" in Chinese?
Interestingly, Chinese language don't use passive voice as explicitly as in English. Translating passive sentences to native Chinese has always been challenging.
One solution is to restore the omitted object, which means:  


  I can lend the book to you once it is returned by C.
  只要C把书还给我，我就能借给你。


However, if the subject cannot be deduced from the context (like, if B doesn't know who borrowed his book), some techniques should be used to make a "passive-less" clause: 


  (lit.) Once the book is returned, I can lend to you.
  只要书还回来，我就能借你。  


Such passive-less sentences are difficult to compare with English, but they are common in Chinese language:  


  砍了这棵树，路就能修成了。
  (lit.) Cut down the tree, and the road can complete.
  The road can be completed once the tree is cut down.  


Note that "passive-less" means sentences without a passive "mark". In Modern Chinese, the passive mark is "被"(bei4). 

P.S. I have found an article explaining the common form of translation from English to Chinese regarding passive sentences. Here are the synopsis:  

Passive sentences in English have been widely used, far more than in Chinese. Therefore, their translations should vary depending on meanings:  


Translate into "judgement" sentences using "是":  



  手语是聋哑人使用的语言。
  Sign languages are (the languages that are) used by the deaf and dumb people.  



Translate with active voice instead of passive voice. If subject is unable to identify, you may need to add some "logical" subjects like "有人"(lit. there are somebody) or "别人"(lit. others):  



  要站着，等有人请你坐下才能坐下。
  (lit.)Need to stand, wait until someone invites you to sit can you sit down.
  Stand until you're invited to sit. 



Translate into subject-less sentences, which are forbidden in English with a few exceptions, but quite common in Chinese:  



  发现了错误就一定要改正。
  (lit.)(If)Discover errors, must correct (them).
  Errors must be corrected once discovered.  



Translate into passive sentences, as what you're familiar with.