Without the requested information, I can only provide you with the following general response, which will hopefully still answer your query. Your rights will depend on whether the offer of an increase in hors was a formally agreed contract between you and the employer, or simply something that was unofficial and still subject to negotiation or formal approval.
If nothing specific or contractual was agreed in writing, it may be difficult to argue your case, especially as the manager can always deny that this was a formal offer and play it down. In that case you would still be entitled to your original contract which you currently hold.
If you are unhappy about this you can raise a formal grievance about it. You can get a detailed explanation of the grievance process here:
In summary, an employee is expected to submit their concerns in writing and send them to their line manager, or whoever is nominated as the person to send grievances to under an official workplace grievance policy.
The complaint should include details of what the grievance is about, any evidence that may exist which is relevant and also what the employee wants their employer to do about this issue.
Once the grievance has been submitted, the employer is expected to arrange a formal grievance hearing, inviting the employee to attend and discuss the nature of their complaint. The meeting is also as an opportunity to ask for further clarification or information, as required.
Following the meeting, the employer will take time to consider all the issues and evidence and then make a decision, communicating it to the employee. If the outcome is not to the complainant’s satisfaction, they can appeal and get a second opinion from a different person assigned by the employer to consider the appeal. Once the appeal is also completed, that brings to an end the formal grievance process and there is no option to escalate it further internally.