If you have a agreement that you are paid a salary plus commission and the work has already
been performed so that you have earned the commission then no, your employer cannot deny your
commission. It sounds like the employer is confusing "commission" with an "incentive". If you made a mistake the employer should formally counsel you, not refuse to pay what you have earned.
Check your employment contract, specifially your explanation of pay to see if there is a salary plus commission clause in your contract.
You can read some examples of commission law suits here http://www.ecouncil.org/commissions.htm


