In your question, it is not God's omniscience which negates free will it the causally determined universe which must exist in order for an omniscient being to be possible. If such a universe were the case, we would not have any free-will as you describe it, whether or not a being existed who knew the outcome. The outcome is already present, known or not. This is essentially the problem of free will as defined by determinists, compatibilists and libertarians, religion or omniscient beings do not really add to the issues.
Modern scientific investigation has limited the strict libertarian argument (simple experiments with sub-concious reaction show that at least some of our actions are outside of our concious control). At a quantum level, scientists are finding difficulty in defining a completely deterministic universe, so it would seem rational, given the current evidence to accept a compatibilist position, that our actions are governed by pre-determined impulses and rational thought processes, but that these seem to us to offer a sufficiently broad range of options as to constitute something we could refer to as free-will.
The only way in which it might be possible for knowledge of this pre-determined action to have any direct impact on the outcome itself would be if the uncertainty of a universe not completely determined was in deed carried by quantum scale particles and such knowledge caused a wave function collapse changing a previously undetermined state into a determined one. This would, however, require a supernatural being as ordinary observers can only have such an effect at a quantum level, scaled-up, the effect disappears.