Nailisi like to lie to people and Saleen knew that long ago. Actually, Nailisi didn't do anything just to deceive people, she just particularly enjoy the extreme mood of human kind, extreme happiness and sadness, from heaven to hell.
In demon dimension, the demon would only hunt for preys when they were hungry but they would taunt them before they eat their preys. What Nailisi did was not at the acme of perfection but it was enough for a man with weak will.
What Saleen did not understand was, why was Nailisi in a hurry to kill the mage. He was a grade six mage! Even though he had a slightly disreputable character, he would be useful if he was kept as of then. If they signed magic contract, he was not afraid that the mage go back on his words.