I would definitely offer her the $3000..... It sucks that she bought the horse from you for $100 and turns around 9 months later wanting to sell it for that much! I had something similar happen in a trade. The horse was an ex-cattle horse that was just a bit much for me. I traded the horse for a supposedly really well broke trail horse, that dumped me every time the wind blew wrong. A few months later I saw the other horse for sale on craigslist. The lady said that she wanted the horse for herself and made it sound like she wasn't going to sell him.
100% Anti-Slaughter and PROUD of it!