If that is the case, can you explain how Buddha knew that Santati will attain arahantship and parinibbana that very day? (“Ānanda, this minister will come to see me this very day and after I have given him a short discourse, he will become an arahat. Soon after becoming an arahat he will realize parinibbāna.”) Here he can predict not only what will become of him, but also when! If there is “free will”, then how can the Buddha be certain that he won’t follow his “free will” and do something different?
What I interpret is that this “free will” is a false perception that we have. However, if one looks at it from the outsider perspective, eg Buddha looking at Santati as an outsider, he can predict what will happen to him. However Santati himself will think that otherwise, that he has the free will to do whatever he wants. Little does he know that his fate is already known! Again, please feel free to correct my interpretation of this.