Without trying to jump to conclusions, from what you describe- this guy sounds like a horse flipper.In which case, he's going to get whatever he can out of you (and he's already gotten a lot). He doesn't care about selling her, his paycheck ends then. You've been reliably paying 500$ a month... whether it's the 1st price or the second, his paycheck ends in less than 3 months- so what good is it to him?
So far as stubborn mare- I was GIVEN a horse like that a couple years back- generally minor health issues- slight arthritis, about 14 years old. She was well trained , just fat, lazy, and had gotten away with throwing fits after being in a pasture for a year. She wasn't mean, not any particular bad habits. I DONATED her to a kid's camp (they were interested) they put 30 days on her (after I slimmed her down), and she was good as new (I wrote that they were to surrender her back to me if she didn't work out for her program)... She is now a lesson horse that does trails for kids. And she was FREE ... FREEEEEE ... free to me, free to them.
This guy you speak of... Shadey, shadey, shadey.