Do teaching load agreements always refer to the academic year?
I've seen this sort of thing happen from time to time; these situations are sufficiently open to interpretation that if you don't have clarity in the letter itself, the chair may try to get you to do a bit of extra teaching. It's entirely reasonable to push back a bit. 

First, though, I'd want to know what you're doing in Spring 2018. If you're getting a semester of teaching release then, that may be just as good as getting in Fall 2017 and thus there would be little reason to argue.

If you're expected to teach in Spring 2018 as well, then you've got a legitimate beef. Just bring it up politely, ask for some remedy, and remember that it's in your department's interest as well as in yours that you have the time you need to get established in your new department and get your research off to a splendid start.