Law of Karma states that we are stuck in a chain. Of action and reaction. Of cause and effect.
Practically like machines. If cause is known, effect can be predicted. What we are till this moment is due to the cause and effect of what we did till this moment. This way we can trace the past to almost beginning of universe.
Are we machines ? Are we operating mechanically ? Where is the free will coming ?
An interesting parallel can be found in Physics. Newtonian laws apply at gross level beautifully.
Whether it is the earth or sun, moon behave according to a specified law. Like machines they perform their roles.
But at micro level, the order breaks down. At sub atomic level, there is only probability and no predictability. The outcome changes depending the observer. This probably is the clue to various miracles attributed to enlightened beings. They are able to witness events at the quantum level and change the outcome.
Any quantum level change will finally affect the gross universe. But by that time a small correction done at quantum level has effected a break in the chain of Karma. Individual has effected his free will. And this free will starts moving him into new cause and effect chain.
Conclusion : Free will operates at the quantum level, Karma operates at the gross level !