Schopenhauer says we are free to act according to motive (which seems to be self evident) but we are not free to choose our motives because these are determined. His argument is very circular and I guess that is the point since he is trying to prove that we cannot establish free will by introspection.
Whether he is a determinist or not (he could be a soft determinist), I certainly reject the idea that all motives are "caused" by something. I think some motives are "willed". This is not to say that there aren't reasons why we have different motives, again that seems to me to be self evident.
Maybe I am redefining will in terms of "reason for acting" rather than "cause for acting". If we look at it that way, we can see how some actions are not "caused" but are the result of a rational process.
If we take an example - someone asks you "why did you kill that fly?"
There are two ways of looking at this question:
1. what is your motive for killing that fly?
2. what is your reason for killing that fly?
On the first interpretation, I accept that the motive is that I found the fly irritating.
However, my reason for killing the fly is not that I found the fly irritating. My reason (in a moral sense) is that I found the fly irritating and I made a moral assessment that I was entitled to end the fly's life because I don't believe that killing flies is wrong in the current circumstances.
My choice to kill the fly was a moral choice, an exercise of free will and it involved reason as well as motive. Anyone who says it isn't, and is just motive, doesn't seem to be capturing what is going on. They stop their analysis too soon.