I'm unfamiliar with US law but that won't stop me from saying you have made a verbal contract (or maybe it's in writing - I don't know) with the owner to buy the mare. There is a possibility that if you back out now and the owner has turned down other offers because you said you would take her there may be the potential for a lawsuit if he cannot get the same amount of money in a subsequently selling (or is it reselling?) of her.
I can definitely see how that would be a problem. I am not sure if the trainer who has the horse right now would be furious about it, because I tried to back out once when I realized her height, and the owner was persistent on wanting ME to be the owner and how she thought WE were a good match. I know she has other current offers on the mare, and a few people want the mare for their children because of her size and easy temper and she really is stunning.