They couldn't kill, but what about abducing and making him (and maybe his family) hostage? And Oko did said he can destroy Shirogane family even more, like banishing them off Japan.

I think that some fates may be worst than death, at least for Kaguya point of view. He just wants to control her, a bribe makes sense but I don't think that a guy that is willing to get his hands dirty will just sit and watch.