My employer said that I can reduce my hours but only once they have filled the rest - can I speed things up as they are taking a long time?

Boss said I can go down to 3 days when they have filled the other 2, but they haven't advertised yet. If I were to hand in my notice they would have to advertise quickly because the job needs doing. Can I do something similar here - effectively hand in my notice for the 2 days I want to drop?