Question: What if an employer does not pay what has been agreed in your contract?Answer:
This is a breach of contract. (See breach of contract)
Try to resolve the issue with your employer directly, if they are uncooperative, or do not believe that they have breached your employment contract, you technically have the legal right to take the matter to court to sue them for breaching the contract. Obviously going to court with your employer is not what you want as this will be a painful, time-consuming and costly exercise, and is only an option when you have first exhausted all manner of informal dispute resolution such as mediation and negotiation.