Attorney Lies and Keeps Client Funds as Alleged "Costs"

If a California contingency attorney were to hypothetically keep a portion of client funds from a settlement check under the guise that the funds were kept to cover "costs" related to litigation but the client can show that those costs were never actually incurred, what legal recourse could the client have? Only recovery of the money improperly kept? Anything above and beyond that and, if possibly so, under what legal theory? Separately, would the state bar take such accusations seriously if reported?

To give a couple examples, suppose the attorney said they expended $X on "expert fees" but they actually never did hire or consult with an expert or they spent $Y on "court filing fees" but they never actually filed anything that cost them any money.