Not a spoiler but my own theory: Isolde knowingly drank poison, she already made a deal with the magician lady (emperor didn't see the last page of her diary where she wrote "i met the magicians of the clock tower. ...spell...work?") and paid it with her life. I think she wished her husband to love her or her husband to be happy or something like that. In either way, emperor probably would love her and be happy if he'd really know her, which he did after she died, so spell worked. So basically spell just made it sure to him to get to know her by making him read her diary, and in order to be happy, he had to turn back in time. That's why magician said she already received the payment, because she already took Isolde's life.
What did the magician lady mean? She said she had already recieved a payment? Any spoilers?(≧∀≦)/