Can paid commissions be taken back?

For this question, services are being sold in the state of CA.

Sales people get paid commissions for services sold and rendered. Can the employer at a later time take back paid commission if for any number of reasons outside the control of the sales person, the client is unwilling to pay for services already rendered? Examples would be, the client did not think the work was executed adequately or the client went out of business (bankrupt), making it difficult to collect. Shouldn’t the sales person be entitled to keep his earned commissions since they did their job and sold the services?