I thought employers could only verify that someone worked for you and that's all.
Lots of people think that. Problem is, they're all wrong. Employers can say anything that is true, that they have a good faith reason to believe is true, or that represents their honest opinion.
There is no legal definition of job abandonment. Job abandonment is what the employer says it is. He is wrong that you MUST stay until September 13 - you can leave any time you want. But if he wants to consider that you abandoned your job if you leave before that, he's allowed to do that. And he's also allowed to tell other employers that.
Bottom line is, while it sounds as if your manager is a real piece of work and you're probably well out of there, he's not doing anything illegal.

