question on termination & regularization of probationary employees

Hi! I would like to ask if a salary increase is mandatory for a newly regularized employee. We hired an employee who is will be 6 months in our company in a couple of days. She was hired to be a project manager. At the time she was hired, she asked for 18k. We could not start her at that salary because it would mean she would earn more than others who have been with the company longer than her. So we verbally agreed on 15k plus 3k allowance. However, her performance during the initial 3 months as project manager was not that good because she was new to the industry so we decided to try her out as project coordinator with the intention of coordinating the works of our production and installation crews. A verbal peer review conducted on her 5th month with the other 5 supervisors she works with was politely not too positive even at that job. Previously,on her 4th month, she asked for a salary increase which will disrupt the salary structure we have because she will actually earn more than her supervisor. We denied this request. Instead, we noticed that she now does overtime work just doing the reports which we feel could and should be done during regular working hours. We believe she is doing this in order to get the salary increase she requested. Now we are a few days to go before her 6th month probationary term. My questions are: 1) should we decide to terminate her, can we do so as just cause based on the verbal peer review? I am not too clear about the probationary ruling on termination. Art. 281 says "The services of an employee who has been engaged on a probationary basis may be terminated for a just cause or when he fails to qualify as a regular employee in accordance with reasonable standards made known by the employer to the employee at the time of his engagement" 2) if we decide to keep her, is it mandatory to increase her salary? we agreed to 18k only when we hired her. As such, we can only afford to keep her at 18k basic and no more allowance. But I believe she will ask for 18k plus allowance . We have to revamp the entire salary structure of the company in this situation which we cannot afford. We are just a small company. We also did not give her a written contract but most of our long term employees also dont have written contracts. Thank you and your quick response is most appreciated.

there is no legal right to an increase. while it will create resentment from her, you are within your rights not to give one