Can God lie?
It was you who was stating that Christ, having no depravity was free,(just as we can make decisions after our salvation, now ofcourse, even this goes against Calvinism since no decisions are made unless it is God making them!) and I was agreeing with what you said (that is right). So which is it now? Was Christ free to say no as well as yes?
As for your question 'can God lie' God cannot lie, but you do not have a correct understanding of His knowledge and how it relates to forordination.
God knew what Christ would do in time and therefore that Plan forordained. Christ did in time what the Father had always knew He would do, hence the Forordination.