Hi there and thank you for your question,
I will try to assist you with your legal question but please feel free to ask as many follow up questions as you like until you are 100% satisfied.
It sounds to me though you have two choices: (1) Give your domestic a warning for not sticking to her agreed upon working hours; or (2) reduce your domestic's working days as a result of your operational requirements.
Re: (1) the warning route -- You'll need to give her 3 warnings before you can dismiss her, but there is nothing stopping you from giving a warning on Monday, Tuesday and Wednesday if she does not stick to the agreed upon working hours for those days. You can then dismiss her on the Wednesday afternoon. If she refers a dispute to the CCMA, you'll need to prove that she has working hours, and that she didn't stick to them, and that she received warnings as a result, and that the third warning was sufficient to dismiss her. You'll need to ensure that her dismissal is both substantially fair and procedurally fair, and for this reason you should read the code of good practice on dismissals here: http://www.ccma.org.za/Display.asp?L1=34&L2=69
Re: (2) reduce your domestic's working days as a result of your operational requirements -- if there are legitimate reasons for reducing an employee's working days/hours, then you can follow the code of good practice on operational requirements (http://www.ccma.org.za/Display.asp?L1=34&L2=67) in order to do this. This will entail an engagement with the employee to discuss it, and to try reach an acceptable solution. If one can't be found, then you are entitled to go ahead anyway with the reduced working days.
Basically, you can achieve what you want to achieve, and there are 2 routes. Perhaps you should discuss the two routes with your domestic and come to an agreement with her. Also, it might assist her if you try to find her work for the other 3 days of the week. After all, she is most certainly worried about her earnings!
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Good luck and best regards,