No, the company is not permitted to withhold such a fee that you have not agreed to pay as this would typically involve a breach of contract claim. Moreover, non-compete agreements are normally illegal in the state of CA, so even if such a fee were required under a contract, it would likely not be enforceable.
Since you were an independent contractor, your recourse would normally involve filing suit against the company for the amount owed in the Superior Court where the company is located.
Because of the amount involved, it would be best to retain a local employment or contracts law attorney to represent you in this matter. A tactfully drafted demand letter from the attorney may be enough to resolve the matter in your favor if the company wishes to avoid a lawsuit.
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