If you had a written commission agreement which set forth targets or goals to get your commission, then the employer must pay you the commission if you hit the written target(s)--they would be contractually obligated to do so, and you could sue them for breach of contract if they fail to do so. When there is a written contract or agreement, the employer must follow it.However, if you don't have a written contract detailing when you get a commission (and how much it would it be), your commission is discretionary, or at the whim of the employer. In that case, the employer could reduce or withhold your commission if they want.