If we assume that God knows the future, we think that the only possible way to do that is because He knows all possible states of the system: He could thus "compute" that future. But this "computation" could maybe imply that no "free will" exists. A SECOND WAY to get that information is hereby proposed: as done by a time-machine, the God's eyes could move in the future and simply OBSERVE it. By doing so a) the above doubts of lack of "free will" are cancelled b) God should commit not to modify at least the part of the future that has been observed: since it has been observed by Him, that future must really take place. Are there concerns about the logical validity of the proposed "second way"?
We don't need the detour via a deterministic universe to get the issue with "free will" and "knowing the future". Provided that your god can reveal information to people, they could tell a person what that person will do tomorrow. The question then becomes whether that person can decide to act differently from what is fortold.
If they cannot, this is a problem for free will.
If they can, you would need something like changing timelines to reconcile. Then you can either have your god being able to observe "future timelines", and repeat the problem; or you can prohibit them from that and get an issue with "omniscience".
 Of course you could have a god that observes the entirety of spacetime, but never acts. Since this god is completely irrelevant, it causes no problems for free will or anything else.