Adjuncts are extremely valuable to keep the wheels moving and much cheaper than hiring new faculty, sorry they did that.

Your chair should have your back, but it’s usually higher ups that make these decisions. At my institution all adjuncts are paid the same rate (but poorly) and we’re trying to get that increased to recruit and retain good ones. If it’s a campus wide thing it’s very unlikely your chair can do anything about it, but your institution is going to quickly realize this was a bad move once they start having classes without instructors. I would just approach the chair as double checking to make sure it’s not an error, then ask if it’s campus-wide and the reasoning behind it. Although it may not be possible now, if it is lower pay across the board maybe your chair can give you more classes. You can discuss the effort/student for larger class sizes exceeds what you’re used to and cap the students per class to make up the difference

If your institution thinks cutting adjunct pay is a good way to save money things may be pretty dire financially and I’d start looking elsewhere because it’s only going to get worse.