Mandarin equivalent of 返 in Cantonese? Is it 回?
As a verb particle, the Mandarin equivalent of 返 in Cantonese is indeed 回

俾返你一啲好處(C) = 給回你一點好處 (M) = give you back some benefit 

Edit:


First, the term 還返 is uniquely Cantonese, "我要還返本書俾佢" in Mandarin would just be "我要还本书给他". 
Second, 本 is a classifier without determiner or count word, it  is normal in Cantonese, but in Mandarin, you should not omit the determiner or count word; you should write "我要还(這)本书给他" or "我要还(一)本书给他" 
Finally, you can omit the classifier along with the determiner and write "我要還書俾佢" / "我要还书给他