Shana, please, please, please deal with the issue of Frosty. The trade should be the LAST thing on your mind. I know you are upset, but like the other poster said, dealing with the trade right now will only make things worse. And you really need to have the herd examined as a whole by a vet. (it doesn't matter where they are or if they have had contact with your other horses or not-don't speculate, just do it and put your mind at ease) If your vet is giving you shady answers, get another vet's opinion. You need to worry about your remaining horses before you worry about getting
more horses. AND...I really hate to be the bearer of bad news but if you go to court over this, the other party will win. She has every legal right to cancel the trade. Yes, there are binding contract laws, but the law will be on HER side when the judge hears that she canceled the deal in concern for her horses. The law only protects you if she backed out for no good reason. I would say she has a very good reason. And this is not to say you take horrible care of your horses, this just means that you need to figure out what is wrong. Halt everything and deal with your current horses. There will be other trades..the world is surely not short of nice minis and once you get things clean and clear again, then maybe you should look into other horses..right now, there is too much going on to worry about MORE.
Hang in there and keep your chin up.
I am really sorry about Frosty and I tried to put this gently, so don't take it the wrong way. It's just an outsider's point of view from a person who has been there. I had trade deals with my breeding rabbits and lost my herd
3 times and lost over $2000 invested in new herds to people who's vets insisted their rabbits were not ill or the illness they had had cleared up. It was gut wrenching, a huge drawn out war and I learned a hard lesson. I got out of rabbits for this very reason. I would hate to see you or the other party end up like this.