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.
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.
Such passive-less sentences are difficult to compare with English, but they are common in Chinese language:
(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":
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):
(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:
(lit.)(If)Discover errors, must correct (them).
Errors must be corrected once discovered.
- Translate into passive sentences, as what you're familiar with.