A few years ago, I was appointed as an acting deputy head during an interregnum. I have just been offered a similar appointment, but the governors are proposing to pay me at a lower point on the scale than they did on the previous occasion. Is this acceptable?
Only you can say, because you have to decide whether to accept it or not. You could, however, very properly take the matter up with the chair of governors and seek an explanation for the change. It could be that they believe the duties are less onerous than they were on the previous occasion. Perhaps the number on roll has fallen significantly or the senior management duties are divided between a larger team.
The governors undoubtedly have the right to determine the salary level at which they offer the post, just as you have every right to turn it down, if you do not wish to do it on the terms proposed.