From what he heard about her, the Saber Fairy had always been the source of all kinds of troubles caused in between the Five Fairies.
After all, it was probably only her who did something like visiting all of her fellow Fairies on the same day and giving then the same offer.
If not for the fact that the Fairies didn't want to create too huge of a scene, the Saber Fairy most likely would've been dead because the suggestion given by her was to kill the Empress and then use her keys along with the keys they all possessed to open the hidden vault.
Hearing all of this Bai Long felt that as long as he roped her in he wouldn't need to do much of anything else.
Given how greedy she was, the Saber Fairy would most likely take care of everything else.
So, as long as he made sure to be careful of the possible betrayal by her, he would come out as the end victor and obtain the rights to see what was inside the vault under the city.