Legally, I can't give advice, and don't know either way.
Since you've already said that, you may have tried these.
What about offering to make your modifications and then make the code available to them? Maybe you don't want that, but there might be some room there.
Or... instead of suggesting a developer license to them, perhaps you could negotiate a fee with them to allow only you to modify it for your own use. That way, you keep your new features to yourself, and they might wind up happy. Don't be surprised if they monitor your site and try to implement the same for their own script. (at least for anything that can be seen on the front end.)
I've seen non-alteration clauses and I've always been able to get permission to modify for my own use.
It's usually best / easiest to get permssion before you buy. That advice is too late for you, but it's meant for others that might read the thread.