Join Date: Sep 2011
Location: Germany- but not German =D
A horse passport isn't proof of ownership, and its easy to obtain a new one.
My suggestion would be that you approach Bob in a friendly manner, recall the conversation and mention anyone who was witness to it, and ask if there is any news or progress with the horse. If he says it wasn't true, then say you didn't lend him your fields for free either, then, and you require the horse for payment.
Bluff him. He might not buy it, but heck, unless you get clever you aren't going to get back at this guy.
After that, I would suggest you take the horse and sell it on yourself, rather than trusting a third party with money and no written contract.
Best of luck.