How much is your horse worth? It sounds like if you do a trade, you will only get $400 cash for her. I personally wouldn't do it. If you need the money get out of her what you can. Adding another horses (even if small) would not be smart if you are getting rid of one for money.
Mini stallions and horses (if not good bloodlines) don't really sell for that much. Just be careful. But, it is totally up to you.